XEbe  XUntverstt^  of  Chicago 


STUDIES  IN  THE  LIFECYCLE 
OFSIMOCEPHALUSVETULUS 

A  Dissertation 
Submitted  to  the  Faculty 

OF  THE 

Ogden  Graduate  School  of  Science 
IN  Candidacy  for  the  Degree  of 
Doctor  of  Philosophy 

Department  of  Zoology 

BY 

WYMAN  REED  GREEN 


Reprinted  from 

The  Biological  Bulletin,  Vol.  XXXVII,  No.  2,  August,  i9i9 


*  private  edition,  distributed  by 

THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 

CHICAGO,  ILLINOIS 

1919 


Digitized  by  the  Internet  Archive 
in  2018  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/studiesinlifecycOOgree 


Cbe  Tlliuversitg  of  Gbicago 


STUDIES  IN  THE  LIFECYCLE 
OFSIMOCEPHALUSVETULUS 


A  Dissertation 
Submitted  to  the  Faculty 

OF  THE 

Ogden  Graduate  School  of  Science 
IN  Candidacy  for  the  Degree  of 
Doctor  of  Philosophy 

Department  of  Zoology 

BY 

WYMAN  REED  GREEN 

'I  I 


Reprinted  from 

The  Biological  Bulletin,  Vol.  XXXVII,  No.  2,  August,  i9i9 


PRIVATE  EDITION,  DISTRIBUTED  BY 

THE  UNIVERSITY  OF  CHICAGO  LIBRARIES 

CHICAGO,  ILLINOIS 

1919 


^ 


IReprinted  from  Biological  Bulletin,  Vol.  XXXVII.,  No  2,  August,  1919  ] 

V 


STUDIES  IN  THE  LIFE  CYCLE  OF  SIMOCEPHALUS 


VETULUS. 

WYMAN  REED  GREEN, 

Hull  Zoological  Laboratory, 

CONTENTS. 

Page. 

I.  Introduction .  49 

11.  Literature  Review .  51 

III.  Material .  56 

IV.  Isolation  Experiments .  57 

1.  Isolation  of  Females  from  General  Cultures .  57 

2.  Experiments  with  Food  and  Culture  Media .  58 

3.  On  Isolated  Stem  Mothers .  62 

4.  Temperature  Experiments .  67 

5.  Miscellaneous  Experiments . 69 

V.  Experiments  on  Ephippial  Eggs .  71 

1.  Evaporation  Experiment  with  NaCl .  72 

2.  Effect  of  Freezing  in  Nature .  72 

3.  Effect  of  Low  Concentrations  of  H2SO4  and  KOH- .  73 

4.  Effect  of  Successive  Freezings .  73 

5.  Effect  of  Injury  or  Removal  of  Ephippia .  74 

6.  Effect  of  Long  Retention  in  Water .  75 

7.  Experiment  on  Eggs  of  Daphnia  pulex .  75 

8.  On  the  Hatching  of  Seventy  Stem  Mothers .  75 

VI.  Observation  on  Pairing .  76 

VI I .  General  Discussion . : .  81 

1.  Relation  of  Males  to  Sexual  States  in  the  Females .  81 

2.  Effect  of  the  Sexual  Phase  on  the  Females .  81 

3.  Relation  of  Age  to  the  Kind  of  Offspring .  82 

4.  Significance  of  Mixed  Broods .  83 

5.  Relation  of  Sexuality  to  Senescence  of  Cultures .  84 

,6.  Effect  of  External  Factors  Upon  the  Life  Cycle .  88 

VIII.  Summary .  89 

IX.  Bibliography .  92 


1.  Introduction. 

These  studies  on  Simocephalus  were  begun  at  the  Hull  Zoologi. 
cal  Laboratory  of  the  University  of  Chicago,  in  1913.  The 
life  cycle  is  very  complex  and  has  not  been  worked  out  satis- 

49 


50 


WYMAX  REED  GREEN. 


factorily  in  all  of  its  phases.  Although  very  creditable  work 
has  been  done  by  Chambers  (’13)  on  spermatogenesis  in  Simo- 
cephalns,  and  by  Weismann  and  a  host  of  others  on  the  natural 
history,  oogenesis,  fertilization,  etc.,  of  the  Daphnians  in  general, 
there  is  still  urgent  need  of  more  data.  Much  time  and  effort 
have  been  expended  by  the  writer  in  the  attempt  to  perfect  a 
technique  which  would  be  adequate  to  the  cytological  problems. 
Considerable  progress  has  been  made  with  this  phase  of  the 
work  yet  the  study  is  far  from  completion.  The  present  paper 
will  be  devoted  to  some  problems  naturally  arising  in  the  course 
of  such  studies,  which  should  be  solved  before  or  at  least  in 
coordination  with  the  cytological  problems,  else  the  latter 
would  lose  much  of  their  significance. 

Among  such  problems  are  the  following:  the  normal  general 
life  cycle  must  be  known;  whether  this  may  not  be  modified  by 
altering  the  external  conditions  and  if  so,  to  what  extent  and 
how ;  whether  the  species  may  not  express  itself  in  all  of  its  forms 
under  each  of  several  sets  of  conditions,  or  if  there  are  certain 
environmental  factors  which  bear  special  relations  to  certain 
forms;  whether  the  failure  to  produce  sexual  forms  during  the 
parthenogenetic  phase  is  due  to  a  deficiency  in  the  environmental 
complex  or  to  internal  conditions  such  as  age  or  is  dependent 
simply  upon  the  general  rate  of  metabolism;  the  normal  propor¬ 
tions  of  males  and  females;  the  ratio  of  sexuab  to  asexual  fe¬ 
males,  the  sequence  of  broods;  the  cause  of  senescence  of  the 
cultures;  the  cause  of  the  appearance  of  the  ephippial  eggs; 
the  time  in  the  ontogeny  at  which  they  appear;  the  precise 
function  of  these  eggs,  i.e.,  whether  the  importance  of  the  ephip¬ 
pial  egg  is  centered  in  the  egg  as  a  means  of  tiding  the  species 
over  unfavorable  seasons  or  periods,  or  in  the  stem  mother  devel¬ 
oping  from  it;  the  relation  of  the  males  to  the  production  and 
development  of  the  ephippial  eggs;  the  normal  length  of  the 
latent  period;  if  it  be  of  definite  duration;  whether  it  can  be 
shortened  and  if  so,  how;  whether  the  offspring  of  a  stem  mother 

^  By  the  term  “sexual  female”  in  this  paper  we  designate  those  females  pro¬ 
ducing  a  series  of  ephippial  eggs  which  require  fertilization;  by  “asexual  female” 
those  which  reproduce  only  parthenogenetically.  All  so-called  sexual  females 
are  destined  to  pass  from  thei:  sexual  phase  into  the  parthenogenetic  phase  and 


so  remain. 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


51 


are  different  from  the  offspring  of  females  produced  partheno- 
genetically;  whether  any  difference  is  to  be  found  bet\veen  the 
offspring  of  parthenogenetic  females  which  have  passed  through 
a  sexual  period  and  those  which  have  not;  whether  a  sexual  fe¬ 
male  may  not  be  retained  in  the  sexual  state,  etc. 

This  paper  presents  data  accumulated  chiefly  during  the  four 
years  from  1914  to  1918  at  Carleton  College  biological  laboratory, 
Xorthfield,  Minn.,  bearing  upon  the  solution  of  these  and  other 
problems  and  definite  conclusions  regarding  some  of  them. 
There  naturally  arises  the  very  interesting  and  important  problem 
of  ascertaining  just  what  correlations  exist  between  the  general 
course  and  the  cytological  aspects  of  the  life  cycle.  The  details 
of  my  work  on  this  latter  phase  of  the  problem  are  to  be  given 
in  another  paper,  now  under  way. 

II.  Literature  Review. 

The  theory  that  there  is  a  sex  cycle  in  Cladocera  which  is 
independent  of  external  factors  was  formulated  by  August 
Weismann.  It  is  now  most  certainly  known  that,  although  the 
correlation  existing  between  environmental  factors  and  the  pro¬ 
duction  of  parthenogenetic  and  sexual  forms  is  not  exact,  certain 
environmental  complexes  do  exist  which  completely  inhibit  the 
appearance  of  males  and  sexual  females. 

A  review  of  the  very  extended  work  of  Weismann  (1877-87) 
and  his  contemporaries  such  as  Kurz  (1875)  and  Schmankewitsch 
(1877),  and  of  later  writers  as  Issakowitsch  (1907),  of  Kuttner 
(1909),  Woltereck  (1909),  Papanicolau  (1911)  and  others, 
leaves  one  very  unsettled  as  to  what  is  the  general  determinative 
principle  underlying  the  course  of  events  in  the  normal  life 
cycle  of  even  the  well  known  and  much  studied  genera  such  as 
Moina,  Daphnia,  and  Simocephalus.  The  Weismannian  con¬ 
ception  of  a  relatively  fixed  “generation  cycle”  for  Cladocera  is 
well  known,  and  has  been  adopted  wholly  or  in  a  modified  form 
by  practically  all  writers  up  to  about  1914,  although  its  truth  had 
been  questioned  by  several  experimenters.  About  this  time 
some  results  were  obtained  by  several  workers  using  various 
species  of  the  three  genera  mentioned,  which  must  be  interpreted 
as  positive  evidence  against  the  theory.  Since  Weismann ’s 


52 


WYMAN  REED  GREEN. 


time  the  recorded  opinions  range  all  the  way  from  unconditional 
acceptance  of  his  extreme  view  to  a  complete  lack  of  faith  in  it. 
For  convenience  of  discussion  these  various  views  may,  in 
general,  be  grouped  under  two  chief  heads: 

I.  That  external  factors  are  of,  little  or  of  no  importance  in 
relation  to  the  succession  of  periods  of  parthenogenetic  and 
sexual  reproduction.  This  was  the  conviction  of  Weismann, 
based  upon  his  researches  extending  over  a  long  period  of  years. 
This  theory  was  broad  enough  to  cover  all  forms  of  life  in  which 
there  is  a  succession  of  parthenogenetic  and  sexual  reproduction. 
Accordingly  the  evidence  for  the  theory  extends  over  a  wide 
field,  and  it  has  numerous  supporters,  having  been  for  a  long 
time  the  orthodox  view. 

This  general  view  is  supported  by  Keilhack  (1906)  and  Ekman 
(1905)  working  on  Polyphemus,  by  Popoff  (1907)  working  on 
various  protozoa  and  metazoa,  and  by  Punnett  (1906)  and 
Whitney  (1907)  working  on  Hydatina,  and  by  others.  The 
last  two  authors  concluded  that  the  age  of  the  strains  is  a  weighty 
factor  in  causing  the  appearance  of  the  sexual  forms.  The  con¬ 
clusions  of  Strohl  (1908)  constitute  a  clean  cut  statement  of  the 
views  up  to  that  time.  Working  with  Polyphemus,  he  concluded 
that  there  was  no  reason  for  abandoning  the  well  grounded 
views  of  Weismann.  Kuttner  (1909)  working  on  Simocephalus 
vetulus  and  other  Daphnians  adopted  Weismann’s  extreme  view. 
McClendon  (1910)  considers  that  ‘‘The  life  cycle  of  a  Daphnid  is, 
therefore  an  heredity  tendency,  but  can  be  influenced  by  nutrition 
and  probably  by  temperature  and  the  accumulations '  of  ex¬ 
cretions,”  and  he  adds  “Nutrition  is  the  most  important  factor.” 

The  extended  experiments  of  Woltereck  (i 909-11)  on  Cladocera 
led  him  to  admit  automaticity  as  one  of  the  factors  in  determining 
the  course  of  their  life  cycle.  Assuming  that  Woltereck’s  cul¬ 
tural  conditions  (1911,  fig.  4,  p.  152)  were  kept  the  same  in  all 
four  cultures  for  the  full  four  years,  one  would  be  forced  to  the 
conclusion  that  there  are  periods  when  environmental  factors 
are  of  no  influence,  and  possibly  other  periods  when  they  are, 
since  the  four  cultures  responded  not  only  differently  in  different 
years,  but  different  cultures  responded  differently  in  the  same 
years;  in  other  words,  the  conclusion  would  seem  to  follow  that 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


53 


there  is  autonomous  variation  in  this  respect,  which  places  the 
whole  question  beyond  the  pale  of  possible  experimental  proof. 
The  careful  experiments  of  Papanicolau  (1911)  merit  more  con¬ 
sideration  than  is  admissible  in  this  paper.  The  views  of  Issa- 
kowitsch  (1907)  and  Scharfenberg  (1911)  agree  with  those  of 
Woltereck  and  Papanicolau  in  that  they  all  assume  certain  in¬ 
herent  tendencies  to  sexuality  which  cannot  be  completely  over¬ 
come  by  any  kind  of  environmental  conditions. 

2.  The  alternative  view  is  that  outer  causes,  such  as  chemical 
substances,  hunger,  temperature,  kind  of  food,  etc.,  are  largely 
or  entirely  responsible  for  the  varying  degrees  of  sexual  and 
parthenogenetic  reproduction.  Evidence  for  this  heterodox 
view  had  begun  to  accumulate  at  least  as  early  as  1875,  when 

Kurz  noted  a  correlation  betw^een  the  drying  up  of  the  water 

/ 

and  the  appearance  of  sexual  forms  in  Cladocera.  Two  years 
later  Schmankewitsch  (1877)  gave  as  his  opinion  that  the  efficient 
cause  is  the  increasing  salt  concentration  due  to  evaporation. 
Among  the  unfavorable  conditions  mentioned  by  Kerherve 
(1892,  p.  236)  poor  nourishment  is  particularly  emphasized  as 
being  responsible  for  the  appearance  of  both  males  and  sexual 
females.  Ostw'old  (1094)  found  temperature  singularly  effective 
as  a  cause  of  sexuality  in  Daphnids.  By  varying  the  temperature 
he  produced  at  the  same  time  all  of  the  forms  that  are  found  in 
nature  at  different  seasons.  Langhans  (1909,  p.  291)  says  that 
Weismann’s  theory  of  a  fixed  generation  cycle  will  not  bear 
critical  examination. 

In  regard  to  other  forms  whose  life  cycle  is  in  general  similar 
to  that  of  Cladocera,  there  is  much  diversity  of  opinion,  although 
evidence  against  a  fixed  internal  “cycle”  is  rapidly  accumulating. 

Finally  we  must  include  under  this  second  category  the  very 
significant  and  definite  conclusions  of  Grosvenor  and  Smith 
(1913)  working  with  Moina  rectirostris,  of  Banta  (1914)  working 
with  Daphnia  pidex,  and  of  Agar  (1914)  working  with 
alus  vetulus,  each  of  whom  has  conclusively  demonstrated  that, 
for  the  form  experimented  on,  there  are  certain  environmental 
complexes  which  will  indefinitely  inhibit  the  appearance  of  males 
and  sexual  states  in  any  of  the  females.  It  is  of  interest  to  note 
that  in  some  instances  the  conditions  which  will  thus  prevent 
the  full  expression  of  the  species  is  quite  narrowly  prescribed. 


54 


WYMAX  REED  GREEX. 


A  third  category  of  opinion  is  sometimes  tacitly  implied  by 
the  manner  in  which  the  problem  of  the  succession  of  forms  in 
Cladocera  is  discussed  by  those  authors  who  believe  that  the 
primary  causes  are  cytological.  Were  this  true  the  problem 
would  not  of  course  be  thereby  removed  from  the  domain  of 
possible  influence  by  environmental  factors.  Here  should  be 
mentioned  the  well-known  ‘‘kern-plasma”  theory  of  Hertwig. 
It  is  supported  by  Papanicolau,  Issakowitsch,  Popoff  and  others.' 

The  results  of  Von  Scharfenberg  and  Papanicolau  have  been 
brought  by  Child  (1915)  into  relation  with  certain  phases  of  his 
general  theory  of  organic  constitution,  as  developed  in  his  book, 
entitled  ‘‘Senescence  and  Rejuvenesence.”  He  says  (p.  391): 
“Von  Scharfenberg  and  Papanicolau  found  that  a  change  in  egg 
character  occurred,  not  only  in  the  course  of  successive  genera¬ 
tions,  but  also  in  the  course  of  single  generations,  i.e.,  the  eggs 
produced  early  in  the  life  of  a  female  are  more  likely  to  develop 
parthenogenetically  into  females  and  those  produced  later  in 
life  into  males  or  to  be  zygogenic  winter  eggs.  In  the  earlier 
generations  of  a  cycle  the  male  producing  and  zygogenic  eggs 
appear  later  in  the  life  of  the  individual,  in  later  generations 
earlier.”  I  am  convinced  that  the  conception  that  zygogenic 
eggs  normally  arise  in  Daphnians  after  the  parthenogenetic  eggs 
is  based  on  entirely  insufficient  evidence.  Definite  statements 
bearing  on  this  point  will  be  found  in  Papanicolau’s  papers 
(1910,  p.  740,  and  1911,  p.  82).  These  views  are  in  substantial 
agreement  with  those  of  IssakoAvitsch  (1907)  and  Scharfenberg 
(1911),  and  diametrically  opposed  to  my  own  since  I  have  never 
found  an  instance  of  the  production  of  ephippial  eggs  following 
a  period  of  parthenogenetic  reproduction.  I  am  convinced  that 
these  conclusions  could  not  have  been  based  upon  observations 
on  isolated  individual  females  of  Simocephalus  vetulus.  Gros- 
venor  and  Smith  (1913,  p.  514)  working  on  Moina  rectirostris 
state:  “We  did  not  find  any  case  of  a  female  that  had  produced 
eggs  parthenogenetically  turning  into  an  ephippial  female.” 

For  several  years  the  writer  was  not  able  to  duplicate  the 
results  of  those  experimenters  who  claimed  that,  under  certain 
conditions,  reproduction  in  some  Cladocera  will  proceed  for  an 
indefinite  number  of  generations  parthenogenetically,  and  that 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


55 


the  change  to  sexual  reproduction,  together  with  the  degenera¬ 
tion  that  so  often  accompanies  it  under  experimental  conditions, 
are  wholly  dependent  upon  the  environment.  Later  experi¬ 
mentation  however  (experiment  5),  has  demonstrated  their 
contention  to  be  correct,  at  least  for  Simocephalus  vetulus. 
Though  these  last  experiments  are  of  a  very  different  type  from 
those  described  by  Agar  (1914&,  Table  I.),  Grosvenor  and  Smith 
(1913),  and  Banta  (1914),  who  succeeded  in  carrying  Daphnians 
through  many  generations  with  no  loss  of  vigor,  they  are  corro¬ 
borative  of  their  results.  That  the  success  of  these  authors  in 
rearing  purely  parthenogenetic  generations  for  an  indefinite 
time  could  not  have  been  due  to  the  accidental  selection  of  lines 
with  a  strong  internal  tendency  to  parthenogenesis  is  rendered 
certain  not  only  by  the  use  of  several  distinct  lines,  but  also  by 
parallel  cultures  subjected  separately  to  unfavorable  temperatures 
and  crowding,  which  gave  a  large  proportion  of  males  and  sexual 
females.  The  genera  used  were  Simocephahis,  Moina  and 
Daphnia.  Banta  carried  Daphnia  pulex  for  127  generations  with 
no  loss  of  vigor. 

In  groups  other  than  Cladocera  in  which  it  has  been  found  that 
an  indefinite  number  of  generations  can  be  reared  parthe- 
nogenetically,  it  seems  that  it  has  always  been  possible  to  bring 
on  sexuality  by  a  proper  change  in  the  environment,  which  would 
indicate  that  pure  lines  with  respect  to  parthenogenesis  do  not 
exist.  It  now  seems  altogether  likely  that  bisexuality  may  be 
indefinitely  inhibited  in  many  more  of  the  lower  animals  than 
experimenters  have  been  aware  of  heretofore.  The  conditions 
under  which  this  is  possible  have  been  found  in  some  instances  to 
be  narrowly  circumscribed,  as  e.g.,  certain  Cladocera  require  the 
high  temperature  of  28°  C.,  although  the  more  recent  authors 
seem  to  believe  that  a  considerable  number  of  species  of  Cladocera 
wiU  be  found  which  can  be  induced  to  reproduce  by  partheno¬ 
genesis  indefinitely  if  only  the  external  conditions  are  properly 
manipulated..  It  appears  also  that,  whatever  may  be  the  ex¬ 
planation,  any  set  of  nearly  uniform  conditions  finally  becomes 
prejudicial  to  continued  parthenogenesis.  Parthenogenetic  re¬ 
production  seems  to  be  favored  by  conditions  which  are  conducive 
to  rapid  growth,  though  they  may  vary  within  certain  limits. 


56 


WYMAN  REED  GREEN. 


Each  species  of  Cladocera  found  capable  of  pure  parthenogenesis 
has  its  own  specific  requirements  as  to  which  environmental 
factors  may  vary  and  to  what  extent  without  it  becoming  zygo- 
genic. 

III.  Material. 

On  taking  up  this  w^ork  much  difficulty  was  encountered  in 
rearing  algae  as  food  for  the  Daphnians.  Numerous  formulae 
were  tried  with  varying  success,  but  for  some  reason  they  did 
not  thrive  on  algae  artificially  produced.  A  simple  and  satis¬ 
factory  solution  of  the  difficulty  was  discovered  in  the  following 
method:  A  number  of  three-  or  four-gallon  aquaria  nearly  full 
of  water  are  placed  where  they  wiU  not  receive  the  direct  rays 
of  the  sun  and  stocked  with  several  kinds  of  unicellular  algae. 
One  to  three  frogs,  depending  upon  the  size,  which  have  been  in 
captivity  until  little  or  nothing  remains  in  the  alimentary  canal, 
are  placed  in  each  of  the  jars.  Of  course  the  aquaria  must  be 
covered,  in  part  to  retain  the  frogs,  but  the  more  important 
reason  for  this  is  to  insure  the  maintenance  of  a  high  percentage  of 
carbonic  acid  gas  and  the  depletion  of  oxygen.  Under  such  con¬ 
ditions  the  algae  grow  rapidly  until  a  condition  of  equilibrium  is 
reached.  When  a  sufficient  quantity  of  algae  has  developed  a 
single  large  Daphnian  with  brood  pouch  full  of  eggs  or  embryos 
may  be  introduced  and  the  frogs  removed,  or  one  may  be  left 
in  with  good  results.  When  the  Daphnians  have  overstocked 
the  culture  they  may  be  strained  out  with  a  cloth  and  trans¬ 
ferred  to  a  new  jar.  The  brown  sediment  of  the  old  culture, 
which  is  excreta  and  dead  algae,  should  be  carefully  removed  so  as 
not  to  loosen  the  algae  on  the  sides  of  the  jar,  new  water  added 
and  more  algae  grown  iii  the  manner  indicated  above. 

The  above  method  has  served  me  very  satisfactorily  for 
Simocephalus  vetulus,  though  it  is  not  so  well  adapted  to  some 
other  forms.  Daphnia  pulex  thrives  in  water  which  is  more  or 
less  filled  with  putrescent  matter.  In  a  pond  frequented  daily 
by  cattle  and  thus  kept  very  roily  and  malodorous,  I  found  this 
species  in  such  numbers  that  by  a  single'  dip  of  the  net  I  secured 
hundreds  of  them.  For  best  results  one  needs  to  work  out  special 
methods  for  each  species.  In  all  of  my  work  I  have  used  Simo¬ 
cephalus  vetulus  unless  otherwise  stated. 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


57 


IV.  Isolation  Experiments. 

In  order  to  ascertain  the  course  of  events  in  the  normal  general 
life  cycle  I  isolated  numerous  specimens,  at  random  at  first,  and 
kept  a  complete  record  of  all  of  their  offspring.  Some  of  these 
will  be  presented  in  detail.  For  convenience  all  experiments 
will  be  referred  to  by  number. 

Experiment  i. — On  June  23  a  very  large  female  with  the  brood 
pouch  full  of  embryos  was  isolated.  On  June  24  a  brood  of  47 
asexual  females  was  extruded.  These  embryos  were  saved  and 
a  first  brood  secured  from  each  of  25  of  them.  The  remainder 
died  early.  There  were  13  pure  broods  of  males,  the  numbers 
in  the  broods  being  as  follows:  4,  3,  1,3,  i,  i,  i,  2,  2,  2,  6,  2,  and  7. 
There  were  6  mixed  broods  as  follows:  3  males  and  6  females, 
8  males  and  4  females,  6  males  and  6  females,  i  male  and  6 
females,  5  males  and  3  females,  and  2  males  and  4  females.  The 
numbers  of  females  in  the  pure  broods  of  females  were:  4,  5,  6,  3, 
4  and  2.  Now  with  this  single  instance  before  us  we  might  well 
ask  why  this  great  variety  in  the  offspring  of  the  members  of  a 
pure  brood  of  47  asexual  females?  But  in  the  light  of  further 
experimentation  along  this  same  line  we  may  reasonably  suppose 
that  had  the  mother  of  these  47  female  embryos  lived  to  produce, 
other  broods,  some  would  have  consisted  of  males  or  at  least 
would  have  contained  males. 

Another  female  isolated  at  the  same  time  lived  until  July  13, 
when  she  died  with  embryos  in  her  brood  pouch.  Her  series 
of  broods  is  as  follows:  i  male  and  6  undetermined,  7  females, 
15  males,  50  females  and  i  male,  15  males,  9  females,  and  13 
females.  The  50  females  of  the  mixed  brood  were  saved  and 
the  first  brood  of  each  of  19  of  them  was  secured.  There  were 
12  broods  of  males  containing  8,  12,  3,  8,  12,  3,  12,  4,  5,  41,  12  and 
6.  There  was  one  mixed  brood  of  12  males  and  females,  and  6 
pure  broods  of  females  contained  6,  10,  i,  3,  2  and  6. 

There  seemed  to  be  no  constancy  as  to  the  ratio  of  males  to 
females  in  the  offspring  of  these  isolated  females,  nor  in  the 
sequence  of  broods,  some  producing  males  first,  some  females, 
and  others  mixed  broods,  in  a  most  capricious  manner.  Hence 
these  miscellaneous  experiments  demonstrated  the  desirability 
of  much  more  extended  experimentation  to  discover  if  possible 
what  order  might  underlie  this  apparent  confusion. 


58 


WYMAX  REED  GREEX. 


Experiment  2. — The  following  experiment  was  designed  to 
show  in  particular  the  normal  ratio  of  males  to  females,  and 
their  sequence,  though  it  furnishes  data  bearing  upon  several 
other  points.  It  was  begun  on  June  23  by  the  isolation  of  a 
single  female  from  a  laboratory  culture.  She  produced  the 
following  broods  on  the  dates  indicated’ 

On  June  26  a  brood  of  43,  27  being  female,  16  not  living. 

On  June  28  a  brood  of  49  females. 

On  July  3  a  brood  of  75  females. 

On  July  7  a  brood  of  7  males. 

On  July  10  a  brood  of  6  females. 

On  July  II  the  isolated  female  died. 

Of  the  brood  produced  on  June  28,  45  which  lived  were  saved 
for  experiment.  These  were  isolated  and  the  sex  of  all  of  their 
offspring  determined,  and  in  several  instances  the  kind  of  females 

i.e.,  whether  sexual  or  asexual.  The  members  of  this  brood  are 
numbered.  The  date  of  the  first  broods  of  each  of  the  first  17 
was  July  4,  of  all  of  the  remainder  it  was  July  5.  The  date  of 
the  death  of  each  female  is  given  at  the  end  of  her  series  of  broods. 
The  date  of  the  last  brood  is  practically  always  not  more  than 
one  day  before  the  death  of  the  female.  The  second  brood  of 
female  number  i  is  a  mixed  brood  and  they  are  so  indicated 
throughout. 

1.  9  females,  5  females  and  9  males,  9  females,  an  undeter¬ 
mined  brood,  July  15. 

2.  9  females,  a  mixed  brood  of  about  20  containing  many 
dead,  6  females,  July  12. 

3.  4  females,  3  males  and  6  undetermined,  ii  males,  4  fe¬ 
males,  July  13. 

4.  4  females,  18  males  and  2  females,  6  females,  i  female, 
July  13- 

5.  3  females,  ii  males  and  8  females,  16  females,  July  9. 

6.  9  males,  3  undetermined,  6  females,  22  females,  July  10. 

7.  7  females,  7  females  and  8  undetermined,  6  females,  5 
females,  an  undetermined  brood,  July  14. 

8.  6  females,  3  females  and  10  undetermined,  10  males, 
July  7,  14. 

9.  8  females,  2  males,  4  males,  July  9. 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS.  59 

10.  6  females,  3  males  and  16  undetermined,  July  7. 

11.  6  males  and  i  undetermined,  4  males  and  i  female,  4 
females,  an  undetermined  brood,  July  13. 

12.  8  males,  3  males,  3  females,  an  undetermined  brood,  July 

9- 

13.  7  males,  i  male,  7  females,  July  9. 

14.  9  females,  a  mixed  brood  undetermined,  6  females,  July 
12. 

15.  6  females,  9  males,  7  females,  July  8. 

16.  10  males,  6  males,  5  females  and  4  undetermined,  10  fe¬ 
males,  July  9. 

17.  9  females,  38  females,  10  males,  July  15. 

18.  12  females,  7  females  and  6  males,  10  undetermined,  7 
females,  July  9.  All  of  the  last  brood  of  7  females  were  asexual 
but  some  of  their  offspring  were  sexual. 

19.  6  females,  10  males  and  14  undetermined,  7  females,  14 
females,  July  9. 

20.  8  females,  21  males,  8  females,  two  other  undetermined 
broods,  July  10. 

21.  An  undetermined  brood,  ii  males  and  12  females  and  14 
undetermined,  July  9. 

22.  7  females,  12  males  and  7  females,  an  undetermined  brood, 
July  9- 

23.  10  males,  2  males  and  13  females,  3  females,  July  13. 

24.  4  males,  6  asexual  females,  10  females,  2  sexual  females, 
15  males,  24  females  of  which  2  were  sexual  and  2  asexual  and  8 
males,  (the  other  20  females  of  the  last  brood  died  too  early  for 
their  sexuality  to  be  determined),  13  females  7  of  which  were 
asexual  and  6  undetermined,  16  asexual  females  and  i  male, 
2  females  i  being  asexual  and  i  sexual,  22  females  12  being  sexual 
and  10  undetermined,  the  next  two  broods  were  accidentally 
left  together  but  their  sum  was  35  females  of  which  12  were  sexual 
and  II  asexual  while  the  remaining  12  were  undetermined,  the 
final  brood  was  7  females  of  which  5  were  sexual  i  asexual  and 
I  undetermined,  August  2. 

25.  5  females,  20  males,  4  females,  July  9. 

26.  One  male  and  5  females,  ii  males  and  2  females,  7  females, 
July  10. 


6o 


WYMAX  REED  GREEN. 


27.  4  males,  i  male  and  4  females,  July  7. 

28.  7  females,  12  males,  3  sexual  females,  7  males,  i  male, 
July  12. 

29.  5  males,  7  males,  6  males,  2  males  and  ii  females,  July  14. 

30.  5  females,  8  males  and  9  undetermined,  July  8. 

31.  4  males,  8  males,  July  7. 

32.  8  females,  16  males,  i  male,  July  9. 

33.  2  females,  22  males,  8  females,  July  9. 

34.  6  males,  7  males,  July  8. 

35.  6  males,  5  males,  July  8. 

36.  8  males,  5  males,  4  females,  July  9. 

37.  8  females,  7  males,  July  8. 

38.  5  females,  22  females,  6  males,  an  undetermined  brood, 
20  females,  i  male  and  35  females,  July  15. 

39.  19  males,  an  undetermined  brood,  15  females,  July  8. 

40.  4  females,  July  7.  (mother  with  abnormal  carapace,  off¬ 
spring  normal) . 

41.  3  males,  3  males,  6  females,  7  males  and  7  sexual  females, 
5  males  and  2  sexual  females,  i  sexual  female,  i  female  and  5 
undetermined,  20  females  15  of  which  are  asexual  the  other  5 
may  be,  15  females,  17  females,  3  females  2  of  which  are  sexual 
and  I  asexual,  2  females  i  of  which  is  asexual  the  other  unde¬ 
termined,  9  females  7  of  which  are  asexual  i  sexual  and  i  un¬ 
determined,  July  31. 

42.  6  males,  2  males,  13  females,  i  male  and  2  females,  July  13. 

43.  5  males  and  2  females,  ii  males,  7  females,  ii  males,  i 
male,  6  females,  ii  males  and  i  female,  2  males,  a  brood  of  fe¬ 
males,  5  females,  an  undetermined  brood,  i  female,  July  26. 

44.  7  males,  8  males,  6  males,  2  males,  10  females,  9  females, 
24  undetermined,  9  females,  8  females,  12  females,  an  undeter¬ 
mined  brood,  4  females,  4  asexual  females,  August  4. 

45.  8  females,  23  males,  an  undetermined  brood,  8  males, 
8  males,  15  males,  10  females,  18  females,  i  male  and  2  females, 
I  sexual  and  i  asexual,  an  undetermined  brood,  7  females, 
July  24. 

Summary  of  Experiment  2. 


The  number  dying  without  offspring .  4 

“  “  possibly  producing  no  males .  3 

“  “  “  “  “  females . 2 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


6l 


The  percentage  of  males  is  about .  42 

The  number  of  times  the  first  brood  was  purely  female .  26 

“  “  “  “  “  “  “  “  “  male .  16 

“  “  “  “  “  «  ‘  ‘  a  mixed  brood .  2 

“  “  . .  “  “  undetermined .  i 

“  “  “  “  last  “  “  purely  female / .  28 

“  “*  “  “  “  .  male .  ii 

“  “  “  ‘‘  ‘  “  “  a  mixed  brood .  6 

The  total  number  of  mixed  broods . • .  .  . .  22 

The  number  producing  no  mixed  broods .  25 

“  “  of  times  the  broods  immediately  preceding  and  follow¬ 
ing  mixed  broods  are  of  the  same  sex .  9 

The  number  of  times  they  are  of  a  different  sex .  4 

The  average  number  in  each  pure  first  brood  of  females .  6.6 

“  “  “  “  “  . males .  7 

“  “  “  mixed  brood .  16 

“  “  of  offspring  per  individual .  39 

“  “  “  “  broods  “  “  .  4-3 

“  “  “  hours  between  the  broods.  . .  45.3 

“  “  female  broods  per  individual .  2.1 

“  “  “  “  male  “  “  “  .  1.3 

“  “  females  in  each  female  brood.  .  .  .' .  4.5 

“  “  “  “  males  “  “  male  “  .  11.9 

“  produced  per  individual .  15.5 

“  “  “  “  females  “  “  “  .  20.7 

“  days  each  of  the  45  females  lived .  14 

The  total  number  of  offspring  was  slightly  over . 1705 


The  45  females  under  consideration  in  this  experiment  were 
divided  into  three  groups.  The  first  group,  consisting  of  24 
individuals,  was  kept  in  water  taken  from  an  ‘‘asexual”  culture 
and  were  well  fed;  the  second  group  of  20  were  also  well  fed  but 
were  kept  in  water  taken  from  a  culture  which  was  producing 
an  abundance  of  males  and  sexual  females;  the  third  group  of 
5  being  reared  in  the  same  kind  of  water  as  the  second  but  were 
poorly  fed.  The  results  of  this  part  of  the  experiment  are 
given  in  the  following  table  in  which  the  data  have  been  reduced 


Group  I 

Group  2 

Group  3 

Average  length  of  life  in  days . 

.  .  .  .  12.4 

II.O 

27.2 

“  number  of  broods . 

-  3.6 

3.0 

10.6 

“  “  “  female  broods ...  . 

. .  .  .  2.1 

1. 1 

5.2 

“  “  “  females  produced . 

...  2  1.6 

12.3 

43.4 

“  males 

.  .  .11.5 

15.6 

29.2 

“  “  “  male  broods . 

...  .9 

1.4 

3-2 

“  “  “  offspring . 

.  .  .37-6 

26.6 

80.6 

% 

% 

% 

Mixed  broods . 

.  ...  9 

10 

II 

62 


WYMAN  REED  GREEN. 


to  the  basis  of  the  individual,  the  data  on  the  first,  second,  and 
third  groups  being  given  in  the  first,  second,  and  third  columns 
respectively,  of  the  table. 

Comparing  these  three  groups  we  find  that  the  differences  when 
averaged  are  not  great  enough  to  be  significant.  The  members 
of  the  third  group  lived  roughly  two  and  one  half  times  as  long 
as  the  others,  and  should  we  reduce  the  data  to  a  common  unit 
of  time  the  correspondence  would  be  very  close.  For  instance, 
the  number  of  broods  produced  by  the  first  group  per  unit  of 
time,  say  27  days,  would  be  10  (2.25  X  3.6  =  10.00),  while  the 
third  group  produces  just  10.6.  The  number  of  'female  broods 
produced  by  the  first  group  would  be  4.72  (2.25  X  2.1  =  4*72), 
as  against  5.2  broods  produced  by  the  third  group,  etc.  As  to 
all  of  the  main  points  under  consideration  the  figures  agree  so 
closely  that  we  must  conclude  that  the  kind  of  water  in  which 
the  three  groups  were  reared  and  the  relative  food  supply  had 
little  or  nothing  to  do  with  the  ratio  of  sexes.  In  the  general 
discussion  of  this  experiment  further  data  are  given  based  on  the 
observation  of  isolated  females  whose  ancestry  is  now  known  for 
one  or  two  generations,  with  conclusions  regarding  the  relation 
of  sexual  to  asexual  females,  males  to  ephippial  egg  production, 
etc.  It  is  deemed  better  however,  to  defer  this  until  after  the 
presentation  of  another  general  isolation  experiment  in  which 
stem  mothers  were  used  instead  of  females  selected  from  the 
general  cultures. 

Experiment  j. — Although  most  of  my  experiments  to  induce 

ephippial  eggs  to  hatch  have  been  failures  I  have  given  them  in 

detail  further  on  in  this  paper  because  of  the  fact  that  so  little 

has  been  accomplished  along  this  line.  I  have  succeeded  in 

securing  about  70  stem  mothers  and  have  had  a  fair  degree  of 

success  in  rearing  them.  All  of  these  were  isolated  at  once  and 

a  complete  record  was  kept  of  the  number  of  broods,  the  kinds  of 

individuals  in  each  brood,  length  of  life,  etc.,  as  shown  in  the 

table  below.  It  was  found  that  a  large  number  died  early,  pro- 

\ 

ducing  few  or  no  offspring.  A  few  however  produced  as  many  as 
9  broods.  The  average  number  of  broods  produced  per  stem 
mother  was  slightly  over  6.  Hence  in  the  tabulation  of  results  no 
account  was  taken  of  stem  mothers  whose  broods  were  too  few. 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


63 


since  these  were  probably  not  perfectly  normal,  else  they  would 
have  lived  longer,  nor  of  broods  beyond  the  sixth  except  to  con¬ 
sider  them  in  determining  the  average  number  of  broods  per 
stem  mother,  since  the  averages  were  not  materially  changed  by 
eliminating  them.  Accordingly  only  24  stem  mothers  were 
considered  fit  to  include  in  the  final  averages,  and  for  similar 
reasons  the  number  of  mothers  in  the  F2  and  F3  generations  were 
reduced  to  16  and  12  respectively.  I  was  much  surprised  to 
note  the  large  proportions  of  males  and  sexual  females  in  the 
broods  of  these  stem  mothers.  This  observation  suggested 
the  experiment  of  rearing  several  generations  to  discover  the 
relative  proportions  of  males  and  sexual  females  in  the  first  and 
later  generations,  to  compare  the  offspring  of  the  stem  mothers 
with  that  of  the  females  produced  parthenogenetically,  and  to 
learn  what  difference,  if  any,  is  to  be  found  between  the  offspring 
of  sexual  females  after  they  have  passed  through  the  sexual  state 
and  of  those  females  which  never  pass  through  this  state.  It 
seemed  that  even  if  passing  through  the  sexual  state  did  not 
affect  the  ratio  of  the  sexes,  or  of  asexual  to  sexual  females,  in 
the  immediate  offspring,  that  the  effect  might  conceivably  be 
cumulative  and  would  be  apparent  in  the  succeeding  generations. 
Hence  the  sexual  and  asexual  females  in  the  Fi  generation  were 
segregated  and  two  distinct  lines,  one  sexual  and  the  other 
asexual,  were  carried  to  the  F3  generation.  Perhaps  the  most 
conspicuous  feature  in  the  records  of  this  experiment  is  one  shown 
only  by  the  individual  records  of  the  females,  namely  that  they 
are  variable  in  the  extreme.  A  given  stem  mother  may  produce 
nearly  all  males,  or  one  kind  of  females,  for  several  broods,  or 
throughout  her  life,  or  they  may  appear  combined  in  all  propor¬ 
tions,  just  as  is  shown  in  the  detailed  individual  records  given  in 
experiment  2,  female  number  41.  The  most  important  facts 
however  are  to  be  deduced  from  the  summary  given  at  the  end  of 
the  tabulated  results.  In  order  to  facilitate  the  comparison  of 
results  in  the  sexual  and  asexual  lines  in  the  second  and  third 
generations,  I  have  placed  the  figures  in  juxtaposition  and  have 
reduced  them  to  a  percentage  basis  at  the  end  of  the  table, 
besides  giving  the  chief  data  in  simpler  form  following  the  main 
summary  of  this  experiment.  Of  course  the  results  in  all  cases 


64 


WYMAX  REED  GREEX. 


are  somewhat  in  error  because  of  the  number  of  offspring  which 
die  too  early  for  identification;  but  the  error  is  not  considerable 
since  all  of  the  available  evidence  points  to  the  conclusion  that 
all  of  the  different  kinds  of  offspring  are  about  equally  viable. 
Inspection  of  this  table  shows  no  marked  changes  in  the  relative 
number  of  kinds  of  offspring  from  the  first  to  the  later  broods  in 
any  of  the  generations  in  either  the  sexual  or  the  asexual  lines. 
This  would  corroborate  the  conclusion  already  reached  from  the 
numerous  other  experiments,  namely  that  sex  is  in  no  way  corre¬ 
lated  with  the  age  of  the  mother.  It  was  of  the  greatest  interest 
to  find  that  the  offspring  of  the  F3  generation  in  the  sexual 
line  were  no  more  likely  to  be  males  and  sexual  females  than  were 
the  offspring  of  the  F3  generation  in  the  asexual  line.  It  seems 
that  if  such  selection  were  to  have  any  definite  effect  that  it 
would  at  least  begin  to  show  by  the  third  generation.  Yet  it 
will  be  observed  that  in  the  second  generation  33.3  per  cent, 
of  the  offspring  in  the  asexual  line  is  male,  and  32.7  per  cent,  is 
male  in  the  sexual  line;  while  in  the  third  generation  the  corre¬ 
sponding  percentages  are  35  per  cent,  and  31.5  per  cent,  respec¬ 
tively.  Of  the  stem  mothers  the  percentage  of  males  in  the 
offspring  is  23.5  per  cent.,  the  lowest  of  all.  It  seems  to  m.e 
however  that  the  differences  in  no  case  are  great  enough  to  be  of 
any  significance  in  a  species  of  animals  which  displays  so  much 
variation  in  so  many  respects  as  this  one  does. 

The  ratio  of  the  asexual  to  the  sexual  females  is  seen  to  be 
27.6  per  cent.:  23.1  per  cent,  in  the  Fi  generation;  19. i  per  cent.: 
15.3  per  cent,  in  the  F2  generation  of  the  asexual  line,  21.9  per 
cent.:  11.3  per  cent,  in  the  F2  generation  of  the  sexual  line;  20.7 
per  cent.:  17.6  per  cent,  in  the  F3  generation  of  the  asexual  line, 
and  28.2  per  cent. :  12  per  cent,  in  the  F3  generation  of  the  sexual 
line.  The  last  ratio  is  striking  as  showing  such  a  low  percentage 
of  sexual  females  in  the  sexual  as  compared  to  the  asexual  lines 
in  the  same  generation. 

It  will  be  seen  that  if  the  percentages  of  the  males  in  these 
five  categories  are  averaged  we  get  somewhat  over  31  per  cent., 
which  is  very  much  lower  than  that  observed  in  experiment  2, 
which  was  42  per  cent.  Since  in  the  latter  case  the  offspring 
considered  numbered  over  1,700,  it  may  seem  that  we  should  be 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


65 


justified  in  concluding  that  the  percentage  of  males  in  the  genera¬ 
tions  immediately  following  the  stem  mothers  is  normally  low. 
Such  a  conclusion  would  be  erroneous,  since  several  experimenters 
have  succeeded  in  maintaining  cultures  of  Simocephalus  and  other 
genera  indefinitely  in  the  parthenogenetic  phase.  In  all  cases 
in  my  experiments  where  females  were  isolated  and  kept  in  small 
containers  in  the  laboratory  some  of  their  broods  were  sure  to 
contain  sexual  forms  if  the  mothers  lived  to  produce  a  reasonable 
number.  In  the  experiment  under  discussion  the  sexual  forms 
did  not  increase  up  to  the  sixth  brood,  after  which  so  large  a 
proportion  of  stem  mothers  died  that  it  seemed  best  to  close  the 
experiment.  However  at  this  point  experiment  5  is  comple¬ 
mentary  since  it  is  concerned  with  a  large  number  of  females 
which  are  of  varying  degrees  of  remoteness  from  stem  mothers 
and  we  find  them  producing  only  about  five  per  cent,  of  sexual 
forms.  It  seems  quite  clear  that  the  remoteness  of  the  generation 
from  the  stem  mother  bears  no  definite  relation  to  the  numerical 
ratio  of  male  to  female  offspring  or  of  sexual  to  asexual  females. 

As  to  the  degree  of  sexuality  of  the  sexual  females  concerned 
in  this  experiment  I  find  no  evidence  that  it  is  different  from  that 
observed  in  the  former  experiment.  The  number  of  ephippia 
produced  can  doubtless  be  taken  as  a  crude  index  to  the  degree  of 
sexuality  of  a  female,  since,  as  shown  elsewhere,  the  number  pro¬ 
duced  is  dependent  upon  factors  inherent  in  the  female,  fertiliza¬ 
tion  of  the  ephippial  egg  haying  no  relation  whatever  to  the 
continuance  of  the  sexual  state.  The  number  of  ephippia  cast 
by  each  sexual  female  in  this  experiment  averages  slightly  less 
than  two.  I  have  found  very  few  females  which  produce  as 
many  as  five  ephippia.  Only  three  instances  are  recorded  in  all 
of  my  experiments.  The  production  of  three  is  common.  Here 
as  in  all  other  cases  observed,  the  sexual  individuals  passed  the 
sexual  state  by  the  time  they  were  about  half  to  two  thirds  grown, 
and  never  returned  to  it  once  they  had  become  asexual.  The 
only  observer  so  far  as  I  am  aware  recording  the  contrary  for 
Simocephalus  vetulus  is  Issakowitsch  (1908).  Moreover,  ephippia 
which  were  barely  noticeable  would  sometimes  appear  and  be 
cast,  asexuality  coming  on  at  once.  Other  individuals  would 
develop  their  ephippia  to  half  or  two  thirds  the  normal  size  and 


66 


WYMAN  REED  GREEN 


ro 

H 

Z 

§ 


Cb 

X 

W 


O 


D 

in 

Q 

H 

*1: 


P 


pa 

< 


H 


I  “5 

D,  U 
P  3 

u 

S 

H-:  ^ 

.  o 

X  hj 

55.^2 

a; 

.£ 
c  J 
c  . 
'Z  X 
cl  1) 

o  ^ 

^  u 
fO  s 

^  .5? 
0^ 

tr* 


D-  S 


c/j  ^bJO 

^  I 

•  O 

X  ^ 

V 

W  1/5 

c’j; 

c 

C 

o  . 

•i:  X 
x  ^ 
u  in 

V 

c  . 

0)  t/j 

O  " 

«s  3 

fa.5f 

O)  IP 

JS  u 
3h  W 


U 

-C 

o 


C^ 


B 


<v 
c 

V 

^  I- 

■St: 

o^ 
b/O  -* 


:/5 

0/ 

C 

tuO 


V 

S 


CJ 


05 

> 

05 

J= 


o 

w 

a 


u 

:/5 

05 

G 


j  ■U.\\.OU'^ 
30U  xsg 

1 

0  0\  uo  00  On 

OoJommOOOmmOOO^A 

M  M 
10  6\ 

•saiBuiaj 

1 

•O'lO  0\  Miort  ■^0\ 

0  MMroPSMOOI'^rooOM  MM 

M 

0  M 

M  <3\ 
M  M 

•S31BIVT 

<50  CN|Mt^l/5MO\  M  sOOO  MM 

'TrOMO^MMMMMrtMlOfOOO  MOO 

M  M 

10 

to  M 

ro  ro 

•S31BlU3j 

jBnx9g 

2 

.9 

1 

0 

1.8 

1.2 

•9 

2 

1-3 

1 

0 

2 

7 

7-1 

1. 1 

1.2 

0 

W  M 

•saj-Eiuaj 

[unxasy 

1 

2 

1-5 

3 

2.1 

.6 

2.1 

5 

•5 

4.2 

•9 

2 

8.1 

16.8 

1-3 

2.8 

20.7 

28.2 

•sib;ox 

lOM  M  lOlO  t^M  MMOOnOONOOs 

M  M  100  o\o  cnm  t^d\ioo6<50'0  Gm  -^^m"  t^M  d\d\dNd\o  d\ 

MM  mMMOO  i-<  OH  MPO^OIOIO 

0  0 
0  0 

M  M 

■uMou;^ 
;ou  X3g 

2.2 

1 

2 

0 

1. 1 

0 

0 

0 

2.1 

0 

•3 

0 

7-7 

I 

1-3 

.2 

16. I 

2.6 

•S3IBUI3  J 

JaqiQ 

10  roioo^  100  roiX)  M 

MMOMMOO  'TtMMMOtMM  MM 

IH 

sO 

10  M 
M  ro 

•sajBjq 

tO'^I>M  MM  vO^  nO 

nOmm  ’rO'^<3‘MO^O>fOl-lMlOM  MM 

M  M 

ro  i> 
ro  c^* 
ro  ro 

•S9[BUI9jJ 

JBnX9g 

0 

.1 

2.1 

1.8 

2.3 

•4 

•3 

0 

.3 

2 

2.2 

0 

7.2 

4-3 

1.2 

.7 

ro  ro 

10  w 

M  M 

'S9[BUI9  ,J 

|Bnx9sy 

00  i>.MrONOi>M  10  ro  lo-^ 

moOmMmm  mm  MOnOO  mm 

M  0\ 

On  G 

M  M 

•SIBIOX 

OM  roMioiOMi>i><0\'OiO'0  0000  rj-ooro 

00000  roM  0  06000  lOMOo  ^00  t^io^o  r^o  '<3‘r'0 

mm  MrororOM  m  ^ooijoO 

0  0 
0  0 

M  M 

*UA\.OU^ 

30U  xag 

ro  On  ro  10  ro 

M  M  M  0  00  M 

ro 

M 

M 

•saiBuia  j 

Jsq;0 

QO  ^  ^  ON 

M  M  M  M  10  Q  M 

M 

T 

10 

0  ^  0 

c^j  ro  H*  0  C3 

M 

10 

ro 

C^l 

*S9[BUI9  j 
[BnX9g 

0  M  00  M  ro 

JM  10  M  M  0  CS 

M 

w 

CO 

•sa[Eui3  j 
[Bnxasy 

1 

4.2 

2.2 

1.6 

6.2 

4-5 

I 

19.7 

3-2 

0 

CN 

•SIBJOX 

■^M  MI>.M  Tj-'^OO  MlOMt^ 

'Ttoodo  iOrj-cj\M'  M  Goo  M 

M  MrOMM  MM  r'OM 

0 

0 

M 

t/i 

V 

'u 

O 

be 

05 

Cl 

^  . 
C 

o 
^  c 
c 


05 

(fi 

05 


•Z 

^  cd 

it:  o 

1/1 


o 

1/1 

■3 

3 


. 

be 

bJO 

be  • 

bfi  • 

biO 

bO 

be  • 

0 

P 

f*  i 

» 

P  • 

_P 

.p  • 

CO 

*n 

'H 

G 

*c 

’H 

"5  ^ 

W 

c. 

c. 

D.  ; 

a 

D, 

a 

X  • 

ex  . 

CO 

CO 

CO 

O} 

CO 

CO 

CO 

fi 

CO 

Jt: 

^  ; 

6:  ■ 

ip 

p 

*S  ; 

p  : 

Cj 

>» 

0 

0 

0  . 

0  . 

0 

0 

_ 

0  . 

<-i-N 

CO 

fi 

U-I 

0 

'-t-l 

0 

0  . 

IH-I 

0  . 

Mh 

0 

Mh 

0 

13  ; 

<4-<i  • 

0  . 

fi 

CO 

CO 

CO 

OT 

CO 

CO 

-S  i 

CO 

0 

b/5 

•fi 

■P 

■p 

■p 

'fi 

0 

^  • 

rH 

CO 

^  ; 

fi  * 

CJ 

•  fi 

CO 

CO 

rs 

fi 

u-» 

IB 

'J4  ' 

P3  . 

24 

2 

'fi 

^  • 

0 

<4-( 

Mh 

Mh 

Mh 

0 

fi 

«4-4 

0 

0 

JZ 

0 

0 

0  • 

0  • 

0 

0 

Cj 

B  * 

0 

Ui 

0 

tH 

tH 

1-, 

CO 

Ui 

CO 

p 

u  * 

■4-i 

0 

0 

0 

CO 

P 

CO 

'C 

0 

0  * 

CJ  w 

P 

P 

<4-4  • 

0 

C 

JZ 

B 

a 

4^ 

M 

rs 

1 

'w 

0 

0 

s 

0  CO 

p 

p  0 

y. 
S  0 

22 

s 

p 

0 

0 

P 

s 

'fi 

0 

0 

"m 

0  , 

rr3 

^  JS 

M-l 

fi 

M 

0 

3  p 

3  >1 

3 

tH 

fi 

u 

P 

c; 

=*  2 

0 

i-4 

fi 

CJ 

4-5 

a 

0 

CJ 

p 

p 

P 

P 

u 

r^ 

s  0 

CJ  ^ 

P  22 
P  Jp 

p 

p  p 

P 

H-) 

bO  • 
fi  riS 

4->  ^ 

<x>  ^ 

V 

tc 

bjQ 

W) 

oe 

CO 

be 

'w 

be  ^ 

buo  -•-> 

bO 

be 

4-> 

Mh 

fi  0 

JZ 

OJ 

fi 

P 

p 

M 

P 

P  ro 

P  P- 

p 

10 

P 

0 

0 

0  0 

p  p 

c 

u 

CJ 

> 

u 

0 

> 

Vh 

CJ 

> 

Vh 

p 

> 

0 

Ph 

P 

> 

<-l-H 

0 

1  0 

§0 

i-i 

p 

> 

<+-1 

0 

P 

> 

<4-4 

0 

s 

0 

^  0 

< 

< 

< 

< 

< 

c 

< 

< 

Ph 

< 

.2 

'm 

03 

a 

bO 

03 

•M 

C 

QJ 

O 

Ph 

(D 

P. 

03 

O 

-i-> 

■P 

<v 

u 

3 

■P 

<Li 

W 

13 

-l-> 

o 

H 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


67 


become  asexual.  After  the  production  of  several  ephippia  a 
partial  one  may  be  developed  as  the  female  leaves  the  sexual 
state,  or  she  may  change  over  abruptly.  The  degrees  of  sexu¬ 
ality  are  infinite,  since  it  is  subject  to  continuous  and  not  to 
integral  variation. 

Considering  the  stem  mothers  from  the  standpoint  of  size 
attained,  viability,  length  of  life,  activity,  etc.,  in  the  first  few 
generations  of  offspring,  in  addition  to  the  proportion  of  the  kinds 
of  offspring  as  to  sex,  and  the  degree  of  sexuality  of  the  females, 
one  is  forced  to  conclude  that  stem  mothers  are  not  functionally 
at  all  unlike  the  females  which  are  produced  parthenogenetically, 
with  the  exception  that  not  a  single  instance  of  the  production  of 
an  ephippial  egg  by  a  stem  mother  has  been  noted.  This  last 
is  in  strict  accordance  with  the  findings  of  Grosvenor  and  Smith 
(1913)  for  the  stem  mothers  of  Moina  rectirostris,  and  of  most 
writers,  though  rarely  instances  of  the  contrary  are  noted,  e.g., 
Scharfenberg  (1911,  p.  24). 

The  most  important  data  are  excerpted  from  the  tabulated 
summary  of  experiment  3  and  slightly  rearranged  in  the  following 

4' 

table  for  the  convenience  of  the  reader. 


Kinds  of  Offspring. 

Asexual 

Fe¬ 

males. 

Sexual 

Fe¬ 

males. 

!Males. 

Other 

Fe¬ 

males. 

Sex  Not 
Known . 

Percentage  of  kinds  of  offspring  (Fi  gen.)  in 
the  first  6  broods  of  24  stem  mothers . 

27.6 

23.1 

23o 

15-4 

II-3 

Percentage  of  kinds  of  o'ffspring  (F2  gen.)  in 
the  first  6  broods  of  16  Fi  asexual  females 

19. 1 

15-3 

33-3 

15.6 

16. 1 

Percentage  of  kinds  of  offspring  (F2  gen.)  in 
the  first  6  broods  of  16  Fi  sexual  females 
after  becoming  parthenogenetic . 

21.9 

II-3 

32.7 

31-2 

2.6 

Percentage  of  kinds  of  offspring  (F3  gen.)  in 
the  first  6  broods  of  12  F2  asexual  females 

20.7 

17.6 

35 

21.6 

5-1 

Percentage  of  kinds  of  offspring  (Fs  gen.)  in 
the  first  6  broods  of  12  F2  sexual  females 
after  becoming  parthenogenetic . 

28.2 

12 

31-5 

19. 1 

9.2 

Experiment  4.  Temperature  experiments  with  Simocephalus 
yield  very  indefinite  results.  Individuals  do  not  thrive  at  a 
higher  temperature  than  28°  C.  Even  at  this  temperature 
isolated  individuals  live  only  a  short  time.  Several  experiments 
designed  to  test  the  effect  of  high  temperature  were  performed 
in  the  following  manner.  Specimens  were  taken  from  laboratory 
cultures  and  placed  in  glass  containers  having  loose  covers  to 


68 


WYMAN  REED  GREEN. 


prevent  excessive  evaporation.  In  each  of  these  experiments 
four  containers  were  used,  of  about  one  pint  capacity,  two  thirds 
full  of  water  at  room  temperature  when  the  Daphnia  were  in¬ 
troduced.  Each  vessel  contained  a  sufficient  quantity  of  green 
algae  for  25  females.  These  containers  were  placed  in  an  electric 
parafin  bath  and  the  temperature  maintained  at  28°  C.  The 
first  one  or  two  broods  produced  under  high  temperatures  were 
found  to  contain  the  usual  proportions  of  males  and  the  two  kinds 
of  females  (see  tabulated  summary  of  experiment  3).  These 
first  broods  were  discarded.  Thus  all  of  the  offspring  in  the 
tabulated  results  of  one  of  these  experiments  given  below,  passed 
their  entire  ontogeny  at  a  temperature  of  28°  C. 


Dates  on  Which  Offspring  were 
Removed. 

Total  Number 
of  Mothers 
Living  on  Dates 
Indicated. 

Asexual 

Fe¬ 

males. 

Sexual 

Fe¬ 

males. 

Offspring 

Males. 

Other 

Fe¬ 

males. 

Sex 

Not 

Known. 

August  25 . 

106 

21 

0 

28 

16 

3 

August  30 . 

102 

14 

12 

30 

42 

17 

September  5 . 

81 

6 

10 

II 

38 

8 

September  7 . 

17 

2 

0 

7 

5 

8 

September  10 . 

3 

0 

3 

2 

0 

4 

September  ii . 

0 

Total  number  of  kinds  of  offspring . 

43 

25 

78 

lOI 

41 

Percentages . 

. 

14.9 

8.7 

27.0 

34-0 

14.0 

| 

Total  number  of  offspring . ^ . 

i 

288 

The  most  important  point  to  be  noted  in  these  results  is  that 
all  kinds  of  offspring  continued  to  appear  to  the  last.  Although 
the  percentages  vary  somewhat,  they  are  within  the  limit  of 
normal  variation.  Other  experiments  were  performed  to  de¬ 
termine  the  effect  of  lower  temperatures.  All  of  the  kinds  of 
offspring  were  secured  at  a  temperature  of  14°  C.,  but  the  general 
metabolism  is  so  much  lowered  at  this  temperature  that  the 
individuals  were  too  few  to  justify  any  conclusions  as  to  the  ratio 
of  the  sexes.  Both  low  and  high  temperatures,  since  they  lower 
the  rate  of  metabolism,  induce  a  decrease  in  the  average  size  of 
the  broods,  in  the  number  of  broods,  viability  of  the  offspring, 
and  shorten  the  lives  of  the  females  used  in  experimenting.  In 
this  connection  it  is  of  interest  to  note  that  females  which  were 
reproducing  parthenogenetically  have  been  obtained  every 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


69 


month  in  the  year  from  shallow  ponds  near  Gary,  Ind.  The 
offspring  obtained  in  the  winter  months  were  asexual  females. 
While  no  sexual  females  and  males  were  obtained  from  eggs  and 
embryos  borne  by  females  at  the  time  of  collecting  from  under 
the  ice,  the  offspring  thus  secured  were  not  sufficiently  numerous 
to  justify  any  statement  as  to  the  production  of  males  and  sexual 
females  at  this  season.  I  am  uncertain  as  to  just  how  far  these 
experimental  results  hold  true  in  nature,  but  there  seems  to  be 
no  reason  for  considering  temperature  a  vital  factor  in  relation 
to  the  sex  cycle  of  Simocephalus  vetulus.  As  mentioned  else¬ 
where,  Grosvenor  and  Smith  (1913)  completely  inhibited  sexual 
forms  in  Moina  rectirostris  at  a  temperature  of  28°  C.,  but  since 
they  could  not  decide  as  to  what  factor  their  success  was  due  it  is 
probably  safe  to  assume  that  they  succeeded  in  spite  of  the  high 
temperature,  not  because  of  it. 

Experiment  5. — The  following  experiment  was  performed  to 
discover  what  kinds  of  offspring  are  produced  under  natural 
conditions.  It  involves  51  females  collected  at  various  times 
from  April  20  to  May  25,  in  the  vicinity  of  Northfield,  Minn., 
from  three  widely  separated  permanent  freshwater  ponds.  The 
sizes  of  the  females  ranged  from  small  (hence  young)  to  very 
large  (hence  old)  individuals.  The  largest  female  among  the 
lot  was  slightly  over  4  mm.  from  the  anterior  margin  of  the  head 
to  the  posterior  end  of  the  carapace  and  2j  mm.  in  vertical 
measurement.  There  is  no  doubt  that  the  age  and  size  of 
Simocephalus  correspond  very  closely.  Though  I  know  the 
pedigree  and  brood  records  of  none  of  these  51  females  it  may 
be  assumed  on  the  basis  of  size  that  some,  such  as  the  smallest, 
had  produced  very  few  broods,  while  others,  the  larger  ones, 
had  produced  very  many,  probably  15  or  20.  Although  the 
individuals  were  not  isolated  there  was  no  crowding.  They 
were  placed  in  two-quart  fruit  jars,  no  jar  containing  more  than 
six  individuals.  Jars  containing  several  specimens  were  kept 
full.  They  were  placed  outside  the  laboratory  windows  where 
the  sun  would  not  strike  them,  being  thus  subject  to  the  ever 
varying  temperatures.  Only  water  brought  *  from  the  ponds 
where  the  females  were  collected  was  used.  This  was  strained 
through  a  fine  silk  cloth  which  removed  all  metazoa,  but  allowed 


70 


WYMAX  REED  GREEX. 


the  passage  of  small  tmicellular  algae.  After  the  first  few  days 
a  gradually  thickening  film  of  green  algae  grew  on  the  sides  of 
the  jars.  To  insure  that  slight  accumulations  of  excreta  and 
other  waste  might  not  affect  them  the  water  was  changed  every 
two  days.  Xo  food  was  provided  other  than  that  which  was 
suspended  in  the  water  and  which  passed  through  the  fine  strainer. 
They  were  thus  subjected  to  the  same  culture  medium,  food  and 
the  same  night  and  day  temperatures,  as  they  were  in  the  natural 
environment.  It  was  thus  hoped  to  duplicate  as  nearly  as  possi¬ 
ble  the  natural  conditions  of  these  ponds  and  to  acquire  positive 
evidence  as  to  the  kind  of  offspring  being  produced  under  natural 
conditions  by  testing  only  the  first  broods  produced  after  the 
females  were  collected.  It  will  be  noted  that  the  tabulated 
presentation  of  this  experiment  gives  the  dates  of  the  collecting 
trips  in  the  first  column,  the  number  of  specimens  secured  on 
each  trip  in  the  second  column,  the  dates  on  which  all  young  were 
separated  from  their  mothers  and  placed  in  other  similar  jars 
in  the  third  column,  the  number  of  offspring  removed  on  each 
date  in  the  fourth,  the  number  of  offspring  which  proved  to  be 
female  in  the  fifth,  etc..  Records  were  kept  of  more  than  one 
brood  from  the  females  collected  on  May  17,  May  20  and  May  25, 
and  it  is  interesting  to  note  that  the  fourth  brood  produced  by 
the  single  female  collected  on  May  17  consisted  of  50  males  and 
one  sexual  female.  In  calculating  the  percentages  of  kinds  of 
offspring  only  those  first  removed  were  taken  into  consideration. 
Of  the  524  offspring  in  this  category  none  is  positively  known 
to  be  male  and  98.8  per  cent,  are  positively  determined  to  coii- 
sist  of  females.  All  of  the  first  offspring  of  the  19  females  col¬ 
lected  on  April  20,  iMay  16  and  May  23  were  saved  to  test  for 
sexuality.  It  is  seen  that  95.6  per  cent.  Avere  asexual,  unless 
possibly  there  were  more  than  7  sexual  females  in  the  brood  of 
77  produced  by  the  single  very  large  female  collected  on  May  16. 
Most  of  this  brood  died  very  early  and  the  mother  died  also 
without  further  progeny.  Excluding  this  one  female’s  offspring 
the  percentage  of  asexual  offspring  of  the  other  18  females  is 
over  99  per  cent.  Xo  sexual  forms,  either  male  or  female,  were 
collected,  but  a  few  ephippial  eggs  were  secured  by  skimming 
the  surface  of  the  ponds  and  these  on  dissection  seemed  to  be 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


71 


perfectly  fresh,  and  could  hardly  have  been  unhatched  eggs  of 
the  preceding  season.  That  they  were  Simocephalus  eggs  is 
quite  certain  since  the  only  other  Daphnian  of  similar  size  in 
these  ponds  is  Daphnia  pulex,  whose  ephippia  contain  two  eggs 
each. 

That  the  practical  freedom  from  sexual  forms  in  this  instance 
is  due  to  external  conditions  is  rendered  absolutely  certain  by 
the  fact  that  these  same  females  when  the  jars  were  placed  in 
the  laboratory  and  the  water  no  longer  regularly  changed,  begun 
to  produce  males  and  sexual  females,  in  just  about  the  same 
proportions  as  in  experiment  3  and  4,  as  nearly  as  could  be  judged 
from  general  observation. 


Tabulated  SujVEviary  of  Experiment  5. 


Date 

Collected. 

Number 
of  Moth¬ 
ers. 

Date  of  Removal 
of  Broods. 

Number 
of  OfT- 
spring. 

Fe¬ 

males. 

Males. 

Asexual 

Fe¬ 

males. 

Sexual 

Fe¬ 

males. 

Sex  Un¬ 
known. 

April  20 

4 

1st  April  28 

33 

33 

0 

33 

0 

0 

May  6 

II 

I  St  May  8 

45 

45 

0 

-> 

0 

May  9 

6 

ist  Maj’  II 

15 

15 

0 

■> 

> 

0 

May  16 

I 

ist  May  17 

77 

77 

0 

7 

IMay  17 

I 

ist  May  20 

4 

p 

■> 

-> 

4 

2d  IVIay  23 

15 

15 

0 

15 

0 

0 

3d  May  25 

45 

45 

0 

45 

0 

0 

4th  May  30 

51 

I 

50 

0 

I 

0 

• 

5th  June  3 

34 

34 

0 

0 

May  20 

6 

1st  May  23 

p 

-> 

-> 

p 

2d  May  26 

120 

118 

2 

0 

May  23 

14 

1st  May  25 

150 

150 

0 

149 

I 

0 

May  25 

8 

1st  May  30 

200 

198 

> 

p 

2 

2d  June  3 

lOI 

97 

I 

> 

3 

Totals . 

51 

890 

828 

53 

242 

9 

9 

Totals  in  ist  broods . 

524 

518 

0 

182 

8 

6 

Percentages  of  kinds  of  offspring  in 

the  1st  broods.  .  .  . 

100 

98.8 

0 

95-6 

4-3 

V.  Experiments  with  Ephippial  Eggs. 

Authors  are  not  agreed  as  to  the  factors  involved  in  shorten¬ 
ing  the  latent  period  of  the  ephippial  egg.  Experimentation 
along  this  line  has  yielded  the  most  inconsistent  results.  I  have 
repeated  the  experiments  of  Weismann  and  others  of  freezing 
the  eggs  for  varying  lengths  of  time  with  all  but  complete  failure. 
Hence  I  shall  give  in  detail  the  experiments  I  have  performed  in 


WYMAX  REED  GREEN. 


my  attempts  to  discover  a  means  of  ending  the  latent  period, 
although  in  not  a  single  instance  have  results  been  all  that  could 
be  desired.  In  a  considerable  number  of  my  experiments  I 
have  made  special  effort  to  reproduce  the  factors  operating  upon 
these  eggs  in  nature,  by  a  series  of  freezings,  alternated  with 
drying,  placing  the  eggs  in  the  sunlight,  etc.,  but  the  results 
seem  to  indicate  that  I  have  neglected  the  one  thing  needful.  Of 
one  thing  I  am  quite  certain,  namely,  that  the  fact  that  the  eggs 
of  Simocephalus  vetuhis  can  be  collected  from  the  fresh-water 
ponds  every  month  in  the  year  must  be  taken  into  account  in 
formulating  any  general  conclusions  as  to  the  factors  concerned 
with  their  development.  Unless  other\vise  stated,  eggs  of 
Simocephalus  vetulus  were  used  in  the  following  experiments. 
In  experiments  performed  with  eggs  collected  from  the  field  there 
were  usually  several  kinds  present  but  those  of  Simocephalus 
predominate.  My  only  successes  have  been  with  Simocephalus 
eggs  which  were  produced  in  the  laboratory. 

Experiment  i. — Since  the  eggs  that  are  laid  in  ponds  are,  as 
the  ponds  drv^  up,  subjected  to  graduallly  increasing  concentra¬ 
tions  of  whatever  salts  may  be  in  solution  in  the  pond  water 
it  occurred  to  me  that  this  might  be  a  potent  factor  in  terminating 
the  latent  period.  To  test  this  I  took  numerous  ephippial  eggs 
from  my  laboratory’  cultures  and  placed  them  in  a  pint  of  pond 
water  and  allowed  it  to  stand  exposed  in  the  laboratory  in  a 
wide  stender  dish.  I  placed  a  graduated  scale  on  the  side  of 
the  dish,  dividing  the  water  depth  into  ten  equal  spaces.  When 
the  water  had  lowered  to  each  of  the  various  levels  by  evapora¬ 
tion,  I  took  out  a  number  of  the  eggs  and  placed  them  in  covered 
vessels.  Continuing  thus  until  the  water  in  the  open  dish  had 
all  disappeared,  I  left  the  last  lot  in  the  open  vessel  until  it 
was  thoroughly  dry.  The  same  experiment  was  tried  starting 
with  low  concentrations  of  XaCl,  but  the  results  were  negative 
in  all  cases. 

Experiment  2. — A  large  number  of  ephippial  eggs  were  col¬ 
lected  from  the  surface  of  the  ice  on  temporary  ponds  near 
Gary,  Ind.,  on  March  12,  1914,  and  dried  at  once.  On  June 
20,  1916,  they  were  placed  in  water  at  room  temperature  and 
left.  They  were  watched  for  several  months  until.it  seemed 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


73 


certain  that  none  would  develop.  On  August  15  the  experi¬ 
ment  was  discontinued.  If  freezing  is  the  most  important  fac¬ 
tor  as  some  authors  have  believed,  it  certainly  cannot  be  all 
sufficient,  for  this  lot  of  eggs  numbered  several  hundreds  and 
they  were  frozen  up  in  the  ice  for  many  weeks  before  they  were 
transferred  to  water.  At  two  later  dates  other  lots  were  col¬ 
lected  and  treated  similarly.  The  last  was  on  March  21,  when 
the  ice  had  begun  to  thaw. 

One  lot  of  eggs  taken  from  a  laboratory  culture  in  April  were 
dried  for  three  days,  then  placed  in  water  at  room  temperature 
but  did  not  hatch.  After  two  months  they  were  taken  out  and 
dried.  Over  two  years  later  they  were  again  transferred  to 
water  at  room  temperature  without  success.  These  experiments 
were  performed  at  the  University  of  Chicago,  but  before  leaving 
there  I  secured  great  numbers  of  ephippial  eggs  by  skimming  the 
surface  of  the  ponds  near  Gary,  Ind.,  and  at  intervals  of  every 
few  months  thereafter  for  three  years  placed  a  number  of  these 
in  water.  None  ever  hatched.  The  eggs  treated  in  this  way 
were  not  counted  though  1,000  would  be  a  very  conservative 
estimate. 

Experiment  j . — This  experiment  was  performed  with  ephippial 
eggs  which  had  been  recently  produced  in  the  laboratory  cul¬ 
tures.  They  were  subjected  to  very  low  concentrations  of  H2 
SO4,  namely,  M/ioo,  M/io,  M/5,  M/2  and  M/i,  for  i,  2,  10,  24 
and  72  hours.  Three  eggs  were  used  for  each  separate  set  of 
conditions.  The  eggs  were  transferred  to  water  and  examined 
daily.  Results  were  negative.  A  precisely  similar  experiment 
was  then  carried  out  with  KOH  in  the  place  of  the  H2SO4,  with¬ 
out  success. 

Experiment  4. — On  January  15,  1914,  10  ephippial  eggs  were 
taken  from  a  laboratory  culture  and  dried  for  three  hours,  then 
kept  in  a  freezing  mixture  for  eight  hours  after  which  they  were 
transferred  to  water  at  room  temperature.  None  hatched. 

On  the  same  date  6  ephippial  eggs  were  transferred  directly 
from  a  laboratory  culture  to  water  which  was  kept  at  30°  C.  for 
four  weeks.  Another  lot  was  kept  at  28°  C.  Results  were 
negative  in  both  cases. 

On  January  18,  1914,  some  ephippial  eggs  which  had  been 


74 


WYMAN  REED  GREEN. 


produced  in  the  latter  part  of  December,  1913,  in  laboratory 
cultures,  were  subjected  to  the  following  conditions:  One  lot  of 
7  was  subjected  to  a  freezing  mixture  for  eight  hours,  one  lot 
of  6  to  a  freezing  mixture  for  eight  hours  on  two  successive  days, 
and  another  lot  of  6  were  given  the  sam_e  treatment  on  three 
successive  days.  In  each  case  the  eggs  were  transferred  from 
the  freezing  mixture  to  water  at  room  temperature.  The  same 
kind  of  treatment  was  applied  to  large  numbers  of  eggs  which 
had  been  collected  by  scraping  the  surface  of  the  ice  on  fresh 
water  ponds  near  Gary,  Ind.,  permitting  the  eggs  to  dry  between 
the  freezings.  Others  were  given  the  same  treatment  and  then 
dried  for  three  months  before  being  transferred  to  water  at  14°  C., 
but  all  to  no  avail.* 

Experiment  5. — During  the  course  of  my  work  I  have  made 
numerous  attempts  to  induce  development  by  clipping  off  the 
small  ends  of  the  ephippia  with  scissors  or  scalpel,  by  pricking 
the  shell  with  a  needle,  and  by  dissecting  it  completely  off.  It 
is  hardly  to  be  supposed  that  such  treatment  would  have  any 
effect  upon  eggs  that  had  never  been  dried,  since  they  often  have 
the  ventral  edges  of  the  ephippia  only  lightly  apposed,  it  being 
possible  in  some  instances  to  look  between  them  upon  the  egg 
inside,  but  conceivably  desiccated  eggs  might  be  thus  influenced. 
It  is  quite  a  simple  matter  to  spread  the  valves  and  set  the  egg 
free.  I  have  succeeded  in  three  instances  in  placing  ephippial 
eggs  with  the  shells  removed  in  the  brood  pouch  of  asexual  fe¬ 
males.  This  did  not  incite  development. 

On  August  14,  1916,  I  dissected  the  shells  from  18  ephippial 
eggs  and  pricked  them  with  an  extremely  fine  glass  needle.  In 
some  cases  I  barely  pierced  the  vitelline  membrane,  and  in  others 
pushed  the  needle  about  one  third  the  way  through  the  egg. 
The  membrane  is  quite  tough  and  in  most  cases  is  under  con¬ 
siderable  tension  due  to  osmotic  pressure.  There  is  a  quantity 
of  liquid  underlying  the  membrane  and  minute  quantities  of 
this  can  be  seen  to  exude  from  the  wound  in  all  cases,  and  if 
the  wound  is  made  with  too  large  a  needle  or  too  deeply  the 
granular  cytoplasm  is  extruded.  Presumably  such  eggs  are 
destroyed.  Fourteen  of  these  eggs  seemed  to  have  been  suc¬ 
cessfully  treated  and  were  placed  in  a  stender  dish  with  enough 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


75 


algae  to  nourish  any  embryos  that  might  appear.  They  were 
examined  daily  for  some  weeks  and  at  intervals  for  two  months. 
Some  of  the  eggs  degenerated  early  and  others  persisted  as  if 
no  injury  had  been  received.  None  had  developed  on  October 
10,  and  shortly  thereafter  no  eggs  could  be  found. 

Experiment  6. — On  January  15,  1916,  two  dozen  eggs  were 
taken  from  a  laboratory  culture  which  had  been  producing  an 
abundance  of  them,  and  without  being  allowed  to  dry,  were 
placed  in  a  vessel  where  the  sun  would  strike  them  for  a  few 
hours  daily,  for  some  weeks.  They  were  then  placed  out  of  the 
sun.  On  August  16  I  placed  them  on  ice  for  24  hours,  and  then 
left  them  until  June,  1917.  That  they  were  in  perfect  condi¬ 
tion  at  this  time  was  shown  by  the  fresh  green  color  of  the  eggs 
on  removal  of  the  ephippia.  They  were  held  under  water  by 
being  placed  in  a  vial  which  was  left  lying  on  its  side  in  a  stender 
dish.  None  of  these  developed. 

Experiment  7. — On  June  30,  1916,  a  number  of  ephippial  eggs 
of  Daphnia  piilex  were  taken  from  a  laboratory  culture  and 
placed  under  water  in  the  same  manner  as  in  the  above  experi¬ 
ment  and  left  thus  submerged  until  May  20,  1917.  Dissection 
of  the  ephippia  from  several  of  these  showed  them  to  be  in  good 
condition  at  the  end  of  this  time,  none  having  developed. 

Experiment  8. — Although  all  of  my  carefully  devised  attempts 
to  induce  ephippial  eggs  to  hatch  by  the  application  of  chemical 
and  mechanical  stimuli,  as  well  as  the  numerous  repetitions 
of  the  freezing  experiments  of  Weismann,  have  completely  failed, 
I  have  succeeded  in  securing  about  70  stem  mothers  from  some 
Simocephalus  vetulus  eggs  which  had  ^  been  given  no  special 
treatment.  On  June  2'j,  1916,  I  removed  several  hundred  eggs 
from  a  culture  which  had  produced  great  numbers  of  them  and 
placed  them  in  a  large-mouthed  8  oz.  bottle  lying  in  a  horizontal 
position  in  a  one-gallon  battery  jar  about  one  third  full  of  water. 
A  small  amount  of  unicellular  green  algae  was  added.  Cypris 
of  several  species  soon  appeared  in  large  numbers.  On  August 
14  an  embryo  of  Simocephalus  appeared.  I  removed  it  in  a 
little  of  the  same  water  together  with  some  of  the  algae.  It 
lived  only  three  days.  Fortunately  others  were  to  follow.  On 
August  18  another  appeared,  and  produced  a  brood  of  7  females 


76 


WYMAX  REED  GREEN . 


on  August  24.  Eight  more  hatched  on  August  21,  7  more  on 
August  25,  10  on  August  26,  6  on  August  30,  4  on  September  3, 
2  September  6,  i  September  20,  and  at  more  or  less  irregular 
intervals  for  several  months  thereafter  others  continued  to 
appear,  until  nearly  six  dozen  had  been  secured. 

Two  other  instances  deseiA*e  mention.  Three  days  before 
setting  up  this  experiment  I  had  placed  a  number  of  eggs,  pro¬ 
duced  from  May  i,  to  June  26,  in  a  jar  in  a  similar  manner  and 
they  remained  there  until  December  12,  when  they  suddenly 
began  to  hatch,  but  only  a  few  appeared.  I  can  suggest  no  ex¬ 
planation  as  to  why  they  did  not  all  hatch,  or  why  the  few  that 
did  should  not  have  done  so  earlier  as  did  most  of  those  men¬ 
tioned  above.  On  August  9  some  fresh  eggs  were  placed  on  ice 
and  left  36  hours  and  then  placed  under  water  in  the  manner 
described  above.  On  December  27  two  hatched.  No  more 
appeared  although  the  experiment  was  left  standing  until  June  i, 
1917,  i.e.,  about  ii  months. 

Mortality  among  these  stem  mothers  was  found  to  be  about 
the  same  as  with  other  females.  I  succeeded  in  obtaining  an 
average  of  a  little  over  six  broods  from  each  of  24  of  these  stem 
mothers  and  have  given  their  history  in  the  interesting  experi¬ 
ment  (number  3)  already  described. 

It  should  be  stated  that  there  were  numerous  other  eggs  in 
the  culture  from  which  the  majority  of  the  stem  mothers  was 
secured,  which  did  not  hatch.  Some  of  the  water  from  this 
culture  was  removed  and  other  newly  produced  eggs  placed  in 
it.  This  experiment  was  left  standing  from  August  30,  1916, 
until  May  5,  1917.  Since  none  of  these  developed  we  must  in¬ 
fer  that  the  kind  of  water  in  the  battery  jar  containing  the 
developing  eggs,  was  not  the  responsible  factor,  though  I  have 
not  the  remotest  idea  what  it  may  have  been. 

VI.  Observations  on  Pairing. 

A  female  from  a  culture  containing  many  ephippial  females 
was  isolated  on  July  19.  By  July  27  she  had  produced  two 
female  broods,  one  of  14  and  another  of  10.  I  placed  the  older 
brood  with  males.  On  August  1st  I  found  that  I4ephippia  had 
been  produced,  though  10  of  them  were  empty,  and  all  but  one  of 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


/  / 

the  females  had  become  asexual.  The  one  sexual  female  re¬ 
mained  so  until  she  had  developed  three  ephippia.  Then  she 
developed  a  brood  of  5  asexual  females  and  died  August  9.  It 
was  not  surprising  to  find  so  large  a  percentage  of  empty  ephippia 
appearing  in  the  presence  of  males  since  the  same  thing  occurs 
in  general  cultures,  though  I  had  thought  it  might,  in  the  latter 
case,  be  due  to  too  small  a  percentage  of  males.*  That  this 
explanation  is  erroneous  is  shown  by  this  and  the  following  ex¬ 
periments. 

Several  of  the  female  broods  obtained  from  the  two  females 
isolated  in  isolation  experiment  number  i,  were  placed  with 
numerous  males  and  examined  daily.  On  July  i  more  males 
were  added  since  I  noticed  several  incipient  ephippia  appearing. 
On  July  30  the  culture  numbered  about  150  individuals,  and  only 
two  ephippia  had  fully  developed,  one  of  these  being  empty. 
By  August  6  there  had  been  developed  14  ephippia,  most  of  them 
empty.  The  percentage  of  ephippial  females  thus  remained 
less  than  10  per  cent.,  so  low  that  it  is  quite  obvious  that  the 
presence  of  males  could  not  be  the  factor  responsible  for  their 
appearance,  since  larger  proportions  of  sexual  females  often 
occur  in  females  which  are  segregated  from  the  males.  Not 
all  of  the  ephippia  reached  full  size,  though  most  of  them  did. 
There  was  a  dearth  of  males  at  no  time  during  this  experiment, 
yet  less  than  half  of  the  fully  developed  ephippia  contained 
fertilized  eggs.  The  fourth  brood  of  female  number  18  of  isola¬ 
tion  experiment  3,  consisted  of  7  females.  These  when  very 
young  were  placed  with  10  males.  All  seven,  however,  proved 
to  be  asexual,  although  some  of  their  offspring  developed  into 
sexual  females  in  the  absence  of  males. 

The  20  females  of  the  fifth  brood  of  female  number  38,  of 
isolation  experiment  2,  were  divided  into  two  lots,  10  being  kept 
with  males  and  10  without.  Of  the  10  with  males,  9  developed 
ephippia,  a  few  of  which  contained  eggs  when  the  molt  was  cast. 
These  eggs,  however  all  degenerated,  so  presumably  they  had 
not  been  fertilized.  The  males  had  all  died  at  this  time.  Six  of 
these  females  became  asexual  after  producing  the  first  ephip- 
pium.  I  placed  some  more  males  with  the  remaining  four  sexual 
females  and  was  fortunate  enough  to  observe  two  matings  of 


78 


WYMAX  REED  GREEX. 


the  same  female  within  two  hours.  I  saw  no  spermatozoa  lost 
in  these  instances  as  I  have  in  several  cases  observed  later,  and 
I  have  no  doubt  that  the  one  normal  ephippial  egg  which  I 
found  later  was  the  result  of  this  pairing. 

Of  the  10  without  males,  one  died  early,  5  developed  ephippia, 
and  4  became  asexual  at  once.  The  5  sexual  females  developed 
in  all  16  ephippia,  which  is  more,  and  more  per  individual,  than 
was  produced  by  those  kept  with  males.  Males  appeared  in 
some  of  the  broods  of  the  asexual  females,  but  most  of  the 
ephippia  were  produced  before  they  appeared,  and  there  was 
only  one  fertilized  egg.  The  sexual  females  all  became  asexual 
before  death. 

The  35  females  of  the  sixth  brood  of  this  same  female  number 
38,  were  also  saved.  The  one  male  of  the  brood  died  at  the 
age  of  four  days.  Of  the  16  ephippia  produced  by  the  members 
of  this  brood  one  contained  an  egg.  Since  the  egg  did  not 
degenerate  it  must  have  been  fertilized.  Thus  we  have  proof 
of  the  early  functioning  of  the  males.  They  are  shorter  lived 
than  the  females,  or  at  least  that  has  been  the  case  with  indi¬ 
viduals  isolated.  It  will  be  noted  that  this  brood  contained  three 
kinds  of  individuals,  namely  asexual  females,  sexual  females, 
and  a  male.  My  records  contain  numerous  instances  of  this 
kind. 

All  of  the  females  of  broods  4,  5,  8  and  ii  of  female  number  41 
of  isolation  experiment  3,  were  placed  with  males  and  a  complete 
record  kept  of  the  results.  The  fourth  brood  consisted  of  7 
males  and  7  females,  all  sexual.  In  all  17  ephippia  were  cast 
by  these  7  females.  Two  of  them  then  died  and  the  remainder 
became  asexual.  In  spite  of  the  presence  of  the  males  15  out 
of  these  17  ephippia  contained  no  egg.  The  fifth  brood  consisted 
of  5  males  and  two  sexual  females.  These  were  reared  together. 
Each  female  produced  a  fertilized  egg  and  one  of  them  an  empty 
ephippium  afterward,  this  one  then  dying.  The  other  became 
asexual  though  the  males  were  continually  present.  The  eighth 
brood  consisted  of  20  females.  A  number  of  males  were  placed 
with  these.  Five  of  the  females  died  early,  and  none  of  the 
rest  developed  even  an  incipient  ephippium.  The  eleventh 
brood  consisted  of  3  females,  2  being  sexual  and  i  asexual.  The 


I 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


79 


asexual  one  was  discarded.  These  two  females  whose  mother 
was  asexual  were  reared  under  conditions  which  were  as  nearly 
alike  as  they  could  possibly  be  made  and  yet  have  them  in  two 
separate  vessels,  gave  rise  to  diverse  offspring.  An  equal  num¬ 
ber  of  males  was  placed  with  each.  The  history  of  one  was: 
an  empty  ephippium,  4  males,  4  sexual  and  2  asexual  females, 
an  undetermined  brood,  death.  The  history  of  the  other  was: 
an  empty  ephippium,  a  fertilized  egg,  a  fertilized  egg,  3  sexual 
and  3  asexual  females,  i  sexual  and  4  asexual  females,  death. 
Each  produces  an  empty  ephippium  at  first,  but  the  former  then 
produces  a  brood  of  4  males,  and  the  latter  another  ephippium, 
this  time  containing  a  fertilized  egg.  Both  later  produce  both 
kinds  of  females.  Several  other  experiments  of  this  same  kind 
gave  similar  results.  I  wish  to  point  out  here  only  one  con¬ 
clusion  based  on  these  experiments,  namely,  that  all  afford 
evidence  that  the  reason  for  the  production  of  male  broods  is 
not  the  production  of  ephippial  eggs  which  were  not  fertilized 
because  of  the  absence  of  males,  for  in  these  experiments  the 
onset  of  male  brood  production  has  been  observed  many  times 
in  the  females  which  have  passed  from  the  sexual  state  and 
begun  the  production  of  males,  in  the  presence  of  other  males. 

The  method  of  copulation  is  interesting.  This  was  observed 
in  several  instances.  The  sixth  brood  of  female  number  44,  of 
isolation  experiment  2,  consisted  of  8  sexual  females  and  i 
asexual  female.  When  the  first  ephippia  of  these  8  sexual  fe¬ 
males  were  in  their  incipiency  I  introduced  5  males  and  watched 
them  almost  continuously,  removing  them  when  I  could  not 
give  them  attention.  When  the  females  were  about  6  days  old 
the  ephippia  were  well  developed  and  the  ephippial  eggs  seemed 
to  be  ready  to  be  extruded  into  the  brood  pouch.  It  was  at  this 
time  that  the  monotony  of  proceeding  was  relieved.  On  being 
placed  in  the  vessel  with  the  females  the  males  at  once  became 
intensely  excited.  This  lasted  nearly  an  hour.  After  several 
abortive  attempts  at  pairing  one  pair  mated.  The  male  opened 
his  carapace  slightly  and  clasped  the  antero-ventral  margin  of 
one  side  of  that  of  the  female.  He  then  bent  his  abdomen  ven- 
trad,  so  that  it  extended  between  the  ventral  margins  of  the 
female’s  carapace,  ventral  to  her  posterior  appendages.  In 


8o 


WYMAX  REED  GREEN. 


this  position  he  waved  his  abdomen  about  for  a  few  seconds, 
then  straightening  it,  he  ejected  the  spermatozoa  in  two  forcible 
streams,  which,  like  puffs  of  smoke,  spread  abruptly  when  their 
force  was  spent.  They  seemed  to  me  to  be  all  swept  away  by 
the  exhalant  respiratory  current  of  the  female.  The  excitement 
of  the  males  was  now  abated,  and  in  about  fifteen  minutes  I 
removed  the  males.  Two  hours  later  I  introduced  the  males 
again  and  secured  a  second  mating.  In  this  instance  the  male 
ejected  the  spermatozoa  just  anterior  to  the  posterior  end  of 
the  abdomen  of  the  female,  and  posterior  to  her  last  pair  of 
appendages.  Again  I  saw  some  of  the  sperm  cells  swept  away, 
but  very  few.  I  am  now  convinced  that  the  first  case  described 
was  abortive.  Other  observations  were  made  later  and  in  some 
cases  no  part  of  the  mass  of  spermatozoa  was  lost,  though  no 
case  was  observed  in  which  the  sperm  cells  were  not  ejected  freely 
into  the  respiratory  chamber  as  described,  the  only  point  of 
contact  being  the  ventral  margins  of  the  male’s  carapace  with 
the  antero-ventral  margin  of  one  side  of  that  of  the  female. 
The  ephippial  egg  is  laid  within  about  an  hour,  and  in  no  case 
observed  more  than  two  hours,  after  the  pairing.  During  the 
period  of  oestruation  the  female  doubtless  produces  some  chemi¬ 
cal  substance  which  passes  out  in  her  exhalent  respiratory  cur¬ 
rent,  since  it  is  by  passing  through  this  that  the  males  are  made 
aware  of  the  females’  readiness  to  mate. 

I  have  made  oft  repeated  attempts  to  observe  the  copulation 
process  from  the  lateral  aspect.  Several  devices,  such  as  a 
trough  made  of  cover  glasses  and  a  slide,  in  which  the  specimens 
were  placed,  yielded  only  failures.  The  males  do  not  always 
attach  to  the  same  side  of  the  female’s  carapace.  In  case  the 
male  attaches  himself  to  the  -left  side  of  the  female’s  carapace, 
the  spermatozoa  are  ejected  near  that  side  of  the  respiratory 
chamber  and  pass  to  the  left  of  her  abdomen  to  the  brood  pouch, 
seemingly  being  driven  dorsad  by  the  action  of  her  last  pair  of 
appendages.  In  one  such  case  observed  the  ephippial  egg  was 
developed  in  the  right  ovary.  In  order  to  enter  the  oviduct 
the  spermatozoa  would  have  to  pass  across  the  brood  pouch  to 
the  right  side.  The  sperm  cells  are  immotile.  Though  I 
have  not  yet  been  able  to  see  them  after  they  have  been  trans- 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


8l 


ferred  to  the  brood  pouch,  I  am  convinced  that  they  are  retained 
in  it  and  that  fertilization  takes  place  after  the  egg  is  laid. 

VII.  General  Discussion. 

1.  What  relation  does  the  presence  of  the  males  bear  to  the 
origin  and  prolongation  of  the  duration  of  the  sexual  state  in 
the  females? 

One  gets  the  impression  from  the  literature  that  the  ephippial 
egg  and  ephippium  are  completely  developed  only  if  fecundation 
occurs.  Perhaps  enough  data  have  already  been  given  in  the 
above  discussion  of  the  observations  on  pairing,  of  the  effect  of 
the  presence  of  males  in  prolonging  the  duration  of  the  sexual 
state  of  the  females  to  convince  any  one  that  it  is  nil,  though  more 
evidence  is  at  hand.  I  wish  to  add  here  that  I  have  observed 
some  hundreds  of  instances  of  embryo  females  developing  through 
the  stage  at  which  they  would  become  sexual,  if  at  all,  which 
never  evinced  the  slightest  evidence  of  sexuality.  In  a  great 
many  instances  these  were  in  the  same  containers  with  sister 
females  which  attained  a  high  degree  of  sexuality.  The  males  used 
in  some  of  these  experiments  were  related  to  the  females,  but  in 
most  instances  they  were  not.  It  is  evident  that  the  ephippial 
egg  and  the  ephippium  are  both  developed  to  within  one  or  two 
hours  of  the  time  of  extrusion  of  the  egg  into  the  brood  pouch, 
in  complete  independence  of  the  male,  the  only  definite  correla¬ 
tions  of  these  processes  with  the  presence  of  males  to  be  noted, 
being  that  the  egg  is  usually  not  laid  in  the  absence  of  fecunda¬ 
tion,  the  exceptions  being,  as  I  should  judge,  about  i  per  cent, 
to  5  per  cent.  I  have  observed  about  half  a  dozen  instances. 

2.  In  the  same  environment  are  individuals  which  pass  through 
the  sexual  phase  any  more  likely  to  give  rise  to  males  and  sexual 
females  than  are  individuals  which  are  parthenogenetic  from  the 
beginning  of  their  reproductive  period? 

Since  the  twenty-fourth  female  in  isolation  experiment  3 
gave  rise  to  pure  broods  of  males,  pure  broods  of  sexual  females, 
pure  broods  of  asexual  females,  to  mixed  broods  of  males  and 
females,  and  to  pure  broods  of  females  of  diverse  sexuality,  thus 
compassing  the  whole  range  of  possibility  as  to  kinds  of  offspring, 
she  was  presumably  normal,  hence  I  used  some  of  her  sexual 


82 


WYMAN  REED  GREEN. 


offspring  with  which  to  test  this  question.  I  selected  12  of  the 
sexual  females  of  broods  12  and  13.  Each  of  these  12,  after 
having  produced  one  empty  ephippium  at  the  age  of  ii  days, 
became  asexual.  Two  of  the  12  died.  In  two  other  cases  the 
first  brood  died  too  early  for  identification,  but  the  first  deter¬ 
minable  broods  of  the  10  that  lived  were  as  follows: 


I. 

6 

females, 

I 

sexual, 

3 

asexual. 

and  2  undetermined 

2 . 

7 

t  i 

2 

3 

“  2 

3- 

3 

0 

2 

“  2 

4. 

3 

0 

3 

5- 

2 

0 

2 

. 

6. 

10 

4 

6 

7- 

6 

I 

5 

8. 

3 

I 

2 

9- 

12 

I 

II 

0. 

6 

males 

Thus  in  these  first  broods  of  the  10  sexual  mothers  only  about 
22  per  cent,  of  the  females  are  sexual,  and  about  12  per  cent,  of 
the  offspring  is  male.  All  of  the  subsequent  broods  of  these 
10  mothers  were  also  saved  and  identified.  It  was  found  that 
the  percentage  of  males  was  slightly  over  8  per  cent.,  a  decrease 
from  that  shown  in  the  broods  immediately  following  the  ephip- 
pia.  This  can  be  of  no  special  significance  however  since  the 
percentage  in  that  case  is  very  much  lower  than  one  usually 
finds  even  in  the  offspring  of  asexual  mothers,  as  noted  in  other 
experiments.  As  we  have  already  seen,  somewhat  over  40  per 
cent,  of  the  offspring,  totaling  over  1,700,  of  the  45  females  con¬ 
cerned  in  isolation  experiment  number  2,  is  male.  About  33 
per  cent,  of  the  broods  of  these  10  sexual  mothers  are  mixed  as 
to  sexuality,  which  is  about  normal.  It  will  be  recalled  also 
that  the  percentage  of  males  and  sexual  females  in  the  sexual 
line  of  offspring  from  the  stem  mothers,  (see  isolation  experi¬ 
ment  3)  is  actually  slightly  less  than  in  the  asexual  line.  It 
thus  seems  certain  that  the  production  of  ephippial  eggs  at  the 
beginning  of  the  reproductive  period,  whether  they  are  fertilized 
or  not,  has  no  influence  on  the  subsequent  offspring. 

3.  Is  the  age  of  the  mother  correlated  in  any  way  with  the 
kind  of  offspring? 

The  twenty-fourth  female  in  isolation  experiment  2,  gave  as 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


83 


her  first  two  broods,  pure  male  offspring.  These  were  followed 
by  three  pure  broods  of  females,  and  then  a  brood  of  15  males. 
The  seventh  brood  was  a  mixed  brood.  Her  last  7  broods,  total¬ 
ing  66,  were  all  females.  The  sexual  and  asexual  females  in 
these  last  broods  occured  in  about  equal  number,  namely,  30 
sexual,  and  36  asexual.  Numberless  such  instances  might  be 
cited  to  show  that  males  often  appear  in  the  first  broods.  The 
summary  of  the  data  in  isolation  experiment  2  shows  that  the 
first  broods  were  pure  male  in  16  instances,  pure  female  in  26 
instances,  mixed  in  2  instances,  and  undetermined  in  i ;  while 
the  last  broods  were  pure  male  in  1 1  instances,  pure  female  in 
28,  and  mixed  in  6.  The  tabulated  results  in  isolation  experi¬ 
ment  3  are  also  in  accord  with  the  data  just  given  and  are,  I 
believe,  quite  conclusive  on  this  point.  It  will  be  noted  that 
there  is  no  marked  increase  or  decrease  in  the  number  of  males; 
sexual  females,  or  of  asexual  females,  from  the  first  of  the  sixth 
broods,  inclusive,  in  the  descendants  of  the  24  stem  mothers, 
for  the  three  generations,  in  either  the  sexual  or  asexual  lines. 
When  the  first  broods  of  newly  collected  old  and  young  females 
are  compared  no  difference  in  kind  is  noted,  as  shown  in  experi¬ 
ment  5.  When  these  same  individuals  are  kept  in  small  con¬ 
tainers  in  the  laboratory,  no  matter  if  the  food  supply  is  abund¬ 
ant,  if  the  water  is  not  changed  often,  sexual  forms  appear  in 
the  offspring  of  young  and  old  alike,  and  in  like  proportions. 
These  facts  point  to  the  conclusion  that  there  is  no  correlation 
between  the  age  of  the  mother  and  the  kind  of  offspring. 

4.  Do  mixed  broods  indicate  a  transitional  stage  from  male 
producing  to  female  producing  and  vice  versa? 

The  summary  of  isolation  experiment  2  shows  9  instances  of 
the  production  of  mixed  broods  Avhich  are  preceded  and  followed 
by  broods  of  the  same  sex,  while  there  are  but  four  cases  in  which 
mixed- broods  are  preceded  and  followed  by  broods  of  different 
sex.  There  are  in  this  same  experiment  two  cases  in  which  the 
first  brood  was  a  mixed  brood,  and  6  cases  in  which  the  last 
brood  was  mixed.  Quite  often  one  mixed  brood  follows  another. 
I  have  found  no  evidence  in  favor  of  the  view  suggested  by  this 
question,  the  occurrence  of  mixed  broods  in  these  experiments 
being  entirely  capricious. 


WYMAX  REED  GREEN. 


84 

5.  What  relation  has  sexuality  to  the  duration  and  senescence 
of  the  laboratory  cultures? 

The  fact  that  my  cultures  which  have  produced  sexual  females 
and  males  have  always  passed  sooner  or  later  into  a  more  or 
less  non-productive  phase  and  did  not  die  out  provided  that  con¬ 
ditions  were  not  so  severe  as  to  terminate  them,  is  well  deserving 
of  consideration,  in  view  of  the  fact  that  one  so  often  reads  in 
Daphnian  literature  of  cultures  becoming  sexual  and  being  there¬ 
by  terminated.  During  the  five  years  I  have  worked  on  Simoce- 
phalus  I  have  run  scores  of  cultures  and  I  have  never  had  an  in¬ 
stance  of  a  pure  sexual  culture  except  those  set  up  by  selecting 
males  and  sexual  females  from  other  cultures.  At  the  most  such 
cultures  remain  purely  sexual  only  a  few  days,  when  with  the 
passing  of  some  of  the  sexual  females  into  the  asexual  phase, 
broods  invariably  appear  containing  asexual  females.  I  have  no 
record  of  a  large  pure  brood  of  sexual  females,  and  no  record  of 
a  female  which  has  consistently  produced  pure  sexual  small 
broods.  Broods  containing  sexual  females  may  be  produced  b}" 
any  female  no  matter  what  her  pedigree  has  been.  Naturally 
if  a  large  percentage  of  the  females  in  a  culture  are  passing  through 
the  sexual  phase,  so  much  of  their  immediate  productivity  is 
sacrificed,  but  since  the  number  of  ephippial  eggs  produced  rarely 
exceeds  4,  and  is  usually  only  i  or  2,  and  always  being  produced 
within  the  first  10  to  25  days  of  their  lives,  every  sexual  female 
devotes  the  most  of  her  life  to  asexual  reproduction.  Old  cul¬ 
tures,  in  which  there  has  been  much  accumulation  of  excreta, 
run  down,  the  size  of  the  broods  decreases  to  from  i  to  5,  many 
individuals  die  before  maturity,  and  the  culture  dies  out  finally; 
but  so  far  as  I  am  able  to  see,  this  neither  causes  nor  results  from 
the  production  of  ephippial  eggs. 

It  is  of  interest  to  consider  what  happens  in  laboratory  cul¬ 
tures  which  are  set  up  and  allowed  to  run  their  natural  course  un¬ 
hindered.  With  each  female  producing  a  brood  of  from  3  to  35 
every  40  or  50  hours,  as  is  always  the  case  in  a  culture  containing 
an  abundance  of  fresh  green  algae  and  no  accumulation  of  excreta, 
any  culture,  even  though  started  with  a  single  individual,  will 
become  overstocked  in  a  few  weeks,  and  the  increase  in  numbers 
has  to  stop,  the  development  of  eggs  in  the  ovaries  and  the 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


85 


growth  of  the  embryos  become  retarded,  the  culture  remains 
nearly  at  a  standstill  for  some  weeks,  and  if  the  food  supply  is 
exhausted  will  finally  die  out,  though  it  has  been  my  experience 
that  they  usually  linger  on  indefinitely.  By  the  time  the  cul¬ 
ture  has  become  overstocked  ephippial  eggs  have  appeared  in 
considerable  numbers,  and  after  having  passed  the  state  of 
equilibrium,  when  the  rate  of  reproduction  has  rather  abruptly 
ceased,  for  some  weeks  just  about  but  not  quite  equaling  the 
death  rate,  the  accumulation  of  ephippial  eggs  may  be  taken  as 
proof  that  the  culture  has  passed  through  a  sexual  phase,  which 
in  turn  is  likely  to  be  erroneously  considered  the  cause  of  the 
decline  of  the  culture.  And  now  with  reproduction  at  its  lowest 
ebb,  and  all  of  the  sexual  females  having  passed  into  the  asexual 
state,  as  they  all  do,  the  observer  is  naturally  likely  to  believe 
that  the  culture  has  passed  into  the  asexual  phase,  since  all  of 
the  sexual  members  of  the  culture  have  yet  to  live  the  asexual 
phase  of  their  lives.  Thus  there  will  be  an  indefinite  period 
when  nearly  every  member  in  the  culture  will  be  asexual.  It 
must  be  remembered  that  at  this  critical  period,  assuming  the 
ordinary  proportion  of  males,  reproduction  must  be  cut  down  to 
somewhat  less  than  two  during  the  life  time,  which  is  a  few  weeks 
or  months,  of  each  female.  Considering  that  25  per  cent,  of 
the  females  are  sexual  (see  table,  isolation  experiment  3),  and 
that  each  sexual  female  is  actually  carrying  an  ephippium  one- 
fifth  of  her  life  time,  even  assuming  that  to  be  but  14  days  it  is 
obvious  that  only  5  per  cent,  of  the  females  will  be  carrying 
ephippia  at  any  given  time.  I  believe  the  life  time  in  a  large 
culture  is  in  reality  much  longer  than  I  have  assumed,  and  25 
per  cent,  far  too  great,  and  that  it  is  safe  to  say  that  in  many 
instances  the  ratio  of  females  actually  carrying  ephippia  at  a 
given  time  is  as  low  as  2  per  cent.  If  it  should  happen  that  these 
few  should  cast  their  ephippia  at  the  same  time,  for  a  day  or  so 
thereafter  one  could  find  a  sexual  individual  only  by  a  tedious 
hunt  with  the  microscope. 

On  the  other  hand,  when  reproduction  proceeds  apace,  starting 
with  10  females  let  us  say,  each  producing  only  6  broods,  with 
an  average  of  i  and  yq  sexual  females  to  a  brood,  (as  was  the 
case  in  isolation  experiment  3),  we  should  have  78  ephippial 


86 


WYMAX  REED  GREEN. 


females  in  the  first  generation  (lo  X  6  X  1.3  =  78) ;  and  suppos¬ 
ing  there  were  in  all  only  5  females  to  a  brood,  (see  table  of  same 
experiment),  we  should  have  300  females  to  mother  the  F2  genera¬ 
tion,  which  should  yield  23,400  ephippial  females,  out  of  a  total 
of  90,000.  Of  course  in  practice,  laboratory  cultures  are  over¬ 
stocked  long  before  all  of  the  second  generation  has  appeared, 
and  in  fact  reproduction  has  practically  ceased  by  the  time  100 
to  200  ephippia  have  appeared.  The  accumulation  of  the  ephip- 
pia  makes  the  culture  seem  to  be  in  the  sexual  phase.  The 
variation  in  the  number  of  kinds  of  offspring  produced  may  de¬ 
pend  to  some  extent  upon  the  extreme  prolificacy  and  conse¬ 
quent  mortality  in  laboratory  cultured,  since  the  assumption  of 
ever  so  slight  a  difference  in  the  susceptibility  of  embryos  to 
the  great  variety  of  adverse  conditions  met  with  in  laboratory 
cultures,  would  be  sufficient  to  account  for  much  of  the  seeming 
variation.  One  may  sometimes  find  several  hundred  ephippia 
in  a  laboratory  culture  in  which  there  are  very  few  males.  In 
one  such  instance  I  collected  200  ephippia,  only  2  of  them  con¬ 
taining  eggs.  Four  explanations  are  possible:  they  may  have 
hatched,  which  is  very  unlikely,  there  may  have  been  too  few 
males  to  fertilize  them,  mating  may  have  been  inhibited  by  some 
unfavorable  cultural  condition,  or,  there  may  have  been  abnormal 
pairing  such  as  I  have  described  in  the  section  on  pairing,  in 
which  case  the  spermatozoa  not  being  retained,  the  eggs  would 
degenerate.  I  have  had  a  few  small  cultures  in  which  for  a  short 
time  males  were  very  numerous  and  sexual  females  almost  ab¬ 
sent,  but  such  instances  are  rare. 

The  following  is  typical  of  my  experience  Avith  Simocephaliis . 
I  isolated  a  female  on  July  i,  1913,  and  kept  her  in  lake  water 
from  lake  Michigan.  This  was  changed  daily.  She  had  nothing 
to  eat  except  what  she  could  get  from  the  lake  water.  This 
seemed  barely  sufficient  since  reproduction  proceeded  very  sloAvly. 
On  August  I  I  placed  6  of  her  descendants  in  a  rich  algae  culture. 
Here  they  multiplied  very  rapidly.  By  August  13  there  were  50 
ephippial  eggs  floating  on  the  surface,  and  males  were  also  present 
in  the  offspring.  This  culture  continued  to  produce  asexual 
forms  for  some  months,  when  the  algae  gradually  died  out  in¬ 
dividuals  in  the  culture  became  less  numerous,  reproduction 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


87 


having  practically  ceased.  Another  culture  was  established 
from  this  by  selecting  a  few  specimens  at  random  and  giving 
them  a  fresh  supply  of  algae.  It  ran  through  a  similar  period  of 
great  productivity,  about  the  same  number  of  sexual  forms  occurr¬ 
ing,  and  declined  as  in  the  first  instance.  A  third  fresh  culture 
was  stocked  by  a  single  specimen  from  the  last.  It  likewise  gave 
rise  to  a  great  profusion  of  Daphnians,  sexual  forms  appearing 
in  considerable  numbers.  Unfortunately  I  did  not  keep  a  record 
of  the  time  it  took  in  the  last  two  instances  for  the  sexual  forms 
to  appear;  but,  as  shown  above,  the  first  culture,  started  August 
I,  produced  50  ephippial  eggs  by  August  13,  and  that  in  the 
presence  of  an  enormous  quantity  of  green  algae,  the  container 
being  a  three  gallon  jar  which  had  previously  been  used  for 
Paramecia.  This  Paramecium  culture  had  been  started  with 
boiled  wheat  as  the  source  of  nutrition  and  had  been  permitted 
to  stand  for  nearly  a  year.  The  Paramecia  had  disappeared  and 
green  algae  had  developed  in  immense  quantities  on  the  side  of 
the  jar  away  from  the  light.  Dearth  of  food  could  not  have  been 
a  factor  in  inducing  sexuality  in  this  culture.  I  have  already 
shown  in  isolation  experiment  2  that  sexuality  comes  on  in  spite 
of  an  abundance  of  food,  in  small  vessels  containing  only  a  single 
individilal  female.  The  study  of  general  cultures  leads  to  con¬ 
clusions  which  are  in  agreement  with  results  obtained  by  a  study 
of  isolated  females,  namely,  that  onset  of  sexuality  is  independent 
of  food  shortage,  and  suggests  that  it  is  related  to  accumulations 
of  certain  excretions,  which  become  critical  in  their  effect  upon 
the  kind  of  offspring  surprisingly  early  in  Simocephalus  cultures 
which  are  allowed  to  run  their  natural  course  unhindered. 
Production  of  ephippial  eggs  reaches  its  highest  level  about  the 
time  a  general  culture  is  over  stocked  and  the  rate  of  reproduction 
has  begun  to  decrease,  passing  to  lower  levels  as  the  food  supply 
decreases,  as  is  readily  demonstrable  if  one  will  but  take  the 
trouble  to  remove  them  as  they  appear  in  such  a  culture,  record¬ 
ing  the  daily  output. 

Of  all  the  females  isolated  for  study  and  kept  in  small  containers 
in  the  laboratory,  including  several  hundred  selected  at  random 
and  as  many  more  whose  pedigree  was  known  for  several  genera¬ 
tions,  also  60  stem  mothers  and  all  of  their  female  offspring  that 


88 


WYMAN  REED  GREEN. 


lived,  for  three  generations,  the  only  instances  in  which  both 
males  and  females  did  not  appear  in  some  of  the  broods  were 
those  in  which  they  were  very  few,  i.e.,  in  which  the  isolated  fe¬ 
males  died  early.  The  same  was  true  of  sexual  females.  They 
always  appeared  in  some  of  the  broods  unless  they  were  too  few. 
Pure  broods  of  sexual  females  w'ere  very  rare  and  they  have  never 
predominated  in  any  of  my  cultures  no  matter  how  severe  the 
conditions  were.  Though  I  have  not  carried  out  a  definite  ex¬ 
periment  to  determine  how  many  generations  a  culture  may  be 
carried  through  the  sexual  females  alone  (except  in  isolation  ex¬ 
periment  3,  where  distinct  lines  were  carried  to  the  third  genera¬ 
tion)  I  have  not  the  slightest  reason  to  suppose  that  they  may 
not  be  carried  on  indefinitely,  since  each  female,  whether  sexual 
or  asexual,  gives  rise  to  both  sexual  and  asexual  females,  provided 
only  that  the  food  supply  and  other  cultural  conditions  have 
proper  attention.  I  am  convinced  that  much  of  the  confusion 
in  the  literature  on  the  relation  of  sexuality  to  the  senescence  of 
cultures  is  due  simply  to  the  extreme  prolificacy  of  the  Daphnians 
and  the  complex  phenomena  resulting  therefrom.  Sexual  phase 
and  asexual  phase  may  well  be  applied  to  individuals,  and  asexual 
phase  to  cultures,  but  in  Simocephalus  vetulus  cultures  never  be¬ 
come  wholly  sexual,  only  partially  so,  since  in  the  most  sexual  of 
cultures  the  offspring  of  a  given  female  are  always  mostly  asexual, 
and  the  sexual  females  pass  most  of  their  lives  in  the  asexual  state. 
Finally  let  us  recall  that  at  the  extremely  modest  rate  of  produc¬ 
tion  of  ephippial  eggs  assumed  in  our  calculations  above,  that 
starting  with  lo  females  we  should  have  23,400  ephippia  at  the 
end  of  the  second  generation,  whereas,  as  a  matter  of  fact  the 
most  sexual  of  cultures  of  Simocephalus  vetulus  produces  only  a 
few  during  a  course  of  many  months.  Hence  it  is  folly  to  con¬ 
sider  the  onset  of  sexuality  causally  related  to  senescence  of  cul¬ 
tures  in  this  species. 

6.  Does  Simocephalus  vetulus  depend  upon  external  factors  to 
call  into  expression  maleness,  sexuality  in  the  females,  and 
parthenogenesis  ? 

With  respect  to  this  point  the  results  of  Grosvenor  and  Smith 
(1913),  Banta  (1914),  and  Agar  (1914b)  are  particularly  instruc¬ 
tive.  They  succeeded  in  completely  inhibiting  sexual  forms  in 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


89 


several  genera  of  Cladocerans  for  an  indefinite  number  of  genera¬ 
tions.  Their  conclusions  are  more  fully  discussed  under  the  head 
of  literature  review. 

In  my  earlier  experiments  there  are  numerous  instances  of 
the  production  of  all  three  kinds  of  individuals,  namely,  males, 
sexual  females  and  asexual  females,  by  the  same  mother.  Less 
common  is  the  production  of  only  two  kinds  of  offspring  during 
the  life  time  of  a  female.  In  these  experiments  I  have  no  record 
of  a  female  producing  only  one  kind  of  offspring  except  those 
whose  progeny  are  very  limited  because  of  the  early  death  of 
the  mother,  and  in  recent  experiments,  those  which  produce 
only  asexual  females,  this  last  being  of  much  importance.  Rarest 
of  all  yet  quite  often  met  with  is  the  production  of  all  three  kinds 
of  offspring  in  the  same  brood.  Female  number  45  of  isolation 
experiment  2  has  one  such  brood  containing  only  three  embryos. 
Broods  of  this  type  are  usually  larger.  The  asexual  females  have 
nearly  always  outnumbered  the  sexual  females  and  males  in 
mixed  broods,  as  they  usually  do  in  the  sum  of  the  broods  of  a 
given  female. 

Similar  proportions  of  the  kinds  of  offspring  may  be  secured 
under  a  great  variety  of  kinds  of  environment.  In  the  most 
highly  sexual  cultures  many  pure  asexual  females  are  always 
present,  and  the  sexual  females  remain  so  only  for  a  brief  period 
in  early  life.  While  these  facts  do  not  prove  that  the  environ¬ 
ment  has  nothing  to  do  with  the  ratio  of  kinds  of  offspring, 
they  would  seem  to  indicate  that  it  is  not  the  determinative 
factor  in  the  nexus  of  causes  that  are  responsible  for  the  expression 
of  the  species  in  all  of  its  forms,  regardless  of  internal  factors. 

VIII.  Summary. 

I.  Life  History. 

I.  The  offspring  of  any  female  of  Simocephalus  vetulus  in  the 
asexual  phase  may  consist  of  any  one  or  all  three  of  the  following 
kinds  of  individuals: 

(a)  Sexual  females,  which  produce  a  series  of  from  one  to 
seldom  more  than  six  ephippial  eggs  early  in  life,  then  become 
parthenogenetic  and  so  remain,  being  then  indistinguishable 
from  other  parthenogenetic  females.  The  final  ephippium  is 


90 


WYMAN  REED  GREEN. 


often  only  partially  developed,  showing  the  sexual  capacity  to 
be  gradually  lost. 

(b)  Parthenogenetic  females,  which  display  no  tendency  to 
produce  ephippial  eggs. 

(c)  Males. 

The  kinds  of  offspring  occur  in  no  definite  order,  but  their 
character  is  probably  determined  at  birth,  and  not  by  subsequent 
conditions.  (See  summaries  of  isolation  experiments  2  and 
3.)  Eggs  which  will  develop  into  males,  into  highly  sexually 
females,  and  parthenogenetic  females,  often  arise  in  the  same 
female  and  not  infrequently  at  the  same  time,  either  early  or 
late  in  her  reproductive  period,  whether  or  not  she  has  passed 
through  the  sexual  state  herself. 

2.  The  sequence  of  the  generations  is  very  indefinite: 

(a)  The  stem  mother  is  functionally  like  the  females  produced 
parthenogenetically,  except  that  she  probably  never  gives  rise 
to  ephippial  eggs.  (However  see  Sharfenberg,  1911,  p.  24.) 
There  is  not  even  an  approximately  definite  number  of  genera¬ 
tions  in  the  cycle  from  one  stem  mother  to  another.  It  may  be 
one  or  many. 

(b)  The  remoteness  of  the  generations  from  the  stem  mother 
bears  no  definite  relation  to  the  percentage  of  males  produced, 
the  ratio  of  sexual  to  parthenogenetic  females,  or  to  the  duration 
of  the  sexual  state  when  present. 

(c)  The  sexual  state  is  probably  determined  in  the  ovary  of 
the  preceding  generation.  There  are  almost  certainly  predispos¬ 
ing  factors  in  the  environment  but  it  is  not  certainly  known  what 
they  are.  Food  or  lack  of  food  does  not  offer  a  sufficient  explana¬ 
tion.  Sexual  females  and  males  tend  to  arise  at  the  same  time, 
presumably  in  response  to  the  same  environmental  complex. 

(d)  Cultures  are  indefinitely  viable  parthenogenetically.  The 
species  will  express  itself  in  all  of  its  forms  under  a  great  variety 
of  conditions.  Under  certain  conditions  sexual  forms  are  com¬ 
pletely  inhibited.  Parthenogenesis  cannot  be  completely  in¬ 
hibited  in  cultures  or  even  in  individual  females.  Thus  cultures 
of  Simocephalus  vetulus  can  never  be  terminated  merely  because 
of  the  onset  of  sexuality. 

{e)  The  production  of  mixed  broods  is  not  to  be  interpreted 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


91 


as  evidence  that  the  female  producing  them  is  undergoing  transi¬ 
tion  from  male  producing  to  female  producing  or  vice  versa. 

3.  The  production  of  ephippia  and  ephippial  eggs  are  related 
but  not  causally,  both  being  dependent  upon  common  internal 
factors.  The  introduction  of  males  into  a  culture  does  not  in¬ 
duce  the  production  of  ephippial  eggs,  nor  does  their  presence 
have  any  relation  to  the  prolongation  of  the  sexual  state  when  it 
has  once  appeared. 

II.  Breeding  Habits. 

Sexual  attraction  is  limited  not  only  to  sexual  females  in  the 
sexual  state,  but  is  confined  to  a  limited  period  of  a  few  hours 
before  the  ephippial  egg  is  laid.  It  seems  to  be  due  to  some  kind 
of  substance  omitted  by  the  female  and  borne  by  her  exhalant 
respiratory  current,  where  it  is  detected  through  a  chemical 
sense  by  the  male.  Fertilization  seems  to  take  place  in  the 
brood  pouch  after  the  egg  is  laid.  The  presence  of  the  spermato¬ 
zoa  in  the  brood  chamber  is  probably  the  stimulus  for  its  extru¬ 
sion.  In  the  absence  of  fertilization  the  ephippial  eggs  are  usually 
resorbed  in  the  ovary,  or,  if  laid  they  undergo  degeneration  in 
the  ephippium  within  one  or  two  days. 

III.  Theoretical. 

The  immediate  significance  of  the  ephippial  egg  is  as  a  stage 
in  the  life  cycle  resistant  to  adverse  conditions,  and  not  in  the 
stem  mother  hatching  from  it,  since  her  offspring  are  in  no  way 
unique.  A  more  remote  but  more  fundamental  significance, 
held  in  common  of  course,  with  all  fertilized  eggs  and  zygotes, 
is  that  it  provides  for  the  permanent  lability  of  the  species 
through  amphimixis.  In  view  of  the  great  prolificacy  of  the 
species  in  regard  to  all  of  its  forms,  and  the  almost  universally 
concomitant  occurrence  of  males  and  sexual  females,  the  develop¬ 
ment  of  only  I  per  cent,  of  the  ephippial  eggs  would  be  quite 
sufficient  to  secure  to  the  species  all  of  the  benefits  to  be  derived 
from  their  two  functions.  It  seems  probable  that  there  is  a 
very  great  inherent  variability  in  the  capacity  for  development  of 
the  ephippial  eggs  in  a  state  of  nature,  and  the  lack  of  uniformity 
of  results  obtained  by  the  various  investigators  in  their  attempts 
to  shorten  the  latent  period  may  be  explainable  on  that  basis. 


92 


WYMAN  REED  GREEN. 


IX.  BIBLIOGRAPHY. 

Agar,  W.  E. 

’14a  Experiments  on  Inheritance  in  Parthenogenesis.  Phil.  Trans,  of  the 
Royal  Soc.,  ser.  B.,  \'ol.  203  and  205. 

’14b  Parthenogenetic  and  Sexual  Reproduction  in  Simocephalus  vetulus  and  other 
Cladocera.  Journal  of  Genetics,  Vol.  III. 

Banta,  A.  M. 

’14  Year  Book,  Carnegie  Inst.  Washington,  XIII.,  p.  141. 

Chambers,  Robert. 

’13  The  Spermatogenesis  of  a  Daphnid,  Simocephalus  vetulus.  Biol.  Bull. 
July. 

ChUd,  C.  M. 

’15  Senescence  and  Rejuvenescence.  Chap.  XIV.  The  University  of  Chicago 
Press.  Chicago,  Ill. 

Cunningham,  Wm.  A. 

’02-3  Studien  an  einer  Daphnide,  Simocephalus  sima,  Beitrage  zur  Kenntnis 
des  Centralnervensystems  und  der  feineren  Anatomic  der  Daphniden. 
Jenische  Zeitschrift  fiir  Xaturvvissenschaft.  Bd.  XXXVII.,  Neue  Folge  30. 
Doncaster,  L. 

’06a  On  Maturation  of  the  Unfertilized  Egg,  and  the  Fate  of  the  Polar  Bodies 
in  the  Tenthredinidae.  Quart.  Jour.  Micr.  Sci.,  49. 

’06b  Spermatogenesis  of  the  Hive  Bee.  Anat.  Anz.,  29. 

’07  Spermatogenesis  of  the  Honey  Bee.  Ibid.,  31. 

Ekman,  S. 

’05  Die  Phyllopoden,  Cladoceren  und  freilebenden  Copepoden  der  nord- 
schwedischen  Hochgebirge.  Zool.  Jahrb.  Abt.  f.  Syst.,  Bd.  21,  pp.  1-69. 
Gilson,  G. 

’84-86  Spermatogenese  chez  les  Arthropodes.  La  Cellule,  Tome  1.  et  11. 
Goldschmidt,  Richard. 

’16  Experimental  Intersexuality  and  the  Sex  Problem.  The  American  Natur¬ 
alist,  Vol.  L.,  p.  705. 

Grabben,  K. 

’79  Die  Entwickelungsgeschichte  der  Moina  rectirostris.  Arb.  Zool.  Inst. 
Wien.,  Bd.  II. 

Grosvenor,  G.  H.,  and  Geoffrey  Smith. 

’13  The  Life-cycle  of  Moina  rectirostris.  Quart.  Journ.  Micr.  Sci.,  Vol.  LVIII. 
Hacker,  Val. 

’94  Die  Entwickelung  der  Winterei  der  Daphniden.  Berichte  d.  Naturf. 
Ges.,  Bd.  VIIL,  Freiburg,  pp.  35-53- 
Hanel,  Elise. 

’08  Vererbung  bei  ungeschlechtlicher  Fortpflanzung  von  Hydra  grisea.  Jen¬ 
ische  Zeitschi.,  Vo).  XLIII. 

Herrick,  C.  L. 

’95  Second  Repoit  of  the  State  Zoologist.  Ser.  II.,.  Nat.  Hist.  Sur.  of  Minn. 
Issakowitsch,  A. 

’07  Geschlechtsbestimmende  Ursachen  bei  den  Daphniden.  Archiv.  f.  Mikr. 
Anat.,  Bd.  LXIX. 

Jennings,  H.  S. 

’08  Heredity,  Variation,  and  Evolution  in  Protozoa,  II.  Proc.  Amer.  Phil. 
Soc.,  Vol.  47. 

Johannsen,  W. 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


93 


’03  Ueber  Erblichkeit  in  Populationen  und  reinen  Linien.  Jena. 

Kammerer,  P. 

’10  Vererbung  erzwungener  Farbveranderungen.  Archiv  fiir  Entwickelungs- 
mech.  Vol.  XXIX.,  p.  457. 

Keilhack,  v.  L. 

’06  Zur  Biologie  des  Polyphemus  pediculus.  Zool.  Anz.,  Bd.  XXX.,  pp.  91 1- 
912. 

Kerherve,  L.  B.  de. 

’92  De  I’apparition  provoquee  des  epphippies  chez  les  Daphnies  (Daphnia 
magna).  Mem.  soc.  zool.  France,  Tome  5. 

’95  Ibid.,  Tome  8. 

Koltzoff,  K.  K. 

’06  Studien  iiber  Gestalt  der  Zelle.  i.  Spermien  der  Dekapoda.  A.  m.  A.,  67. 

Kuhn,  Alfred. 

’08  Archiv.  f.  Zell  Forschung,  Bd.  L,  p.  538. 

’ll  Zool.  Anz.,  Bd.  XXXVIII. 

Kurz,  V.  Wilhelm. 

’75  Dodekas  neuer  Cladoceren  nebst  einer  kurzen  libersicht  der  Cladoceran- 
fauna  Bohmens.  Sitz.  Ber.  math,  naturw.  Wien. 

Kuttner,  Olga. 

’09  Untersuchungen  iiber  Fortpflanzungsverhaltnisse  und  Vererbung  bei 
Cladoceren.  Intern.  Rev.  d.  ges.  Hydrobiol.  und  Hydrogr.,  Bd.  II. 

Labbe,  A. 

’03  Sur  la  spermatogenese  des  crustaces  decapodes.  Compt.  rend.  Acad.  d.  s. 
Par.,  CXXXVIL,  pp.  272-274. 

’04  La  maturation  des  spermatids  et  la  constitution  des  spermatozoides  chez 
les  crustaces  decapodes.  Arch,  de  zool.  exper.  et  gtn.  Hist,  naturelle,  etc.. 
Par. 

Langhans,  V.  H. 

’09  Ueber  experimentelle  Untersuchungen  zu  Fragen  der  Fortpflanzung, 
Variation  und  vererbung  bei  Daphniden.  Veih.  d.  deutsch  zool.  Gesellsch. 

Lebedinski,  J. 

’91  Entwickelung  der  Daphniden  aus  dem  Somerei.  Zool.  Anz.,  Bd.  XI\’., 
p.  149. 

Leydig,  Franz. 

’60  Naturgeschichte  -der  Daphniden.  Zeitschrift  f.  Wiss.  Zool.,  Bd.  III., 
Tubingen. 

Lubbock,  Sir  John. 

’57  An  Account  of  the  Methods  of  Reproduction  in  Daphnia  and  the  Structure 
of  the  Ephippium.  Phil.  Trans,  of  the  Royal  Soc.  of  London,  p.  57. 

Maupas,  E. 

’91  Sur  la  Determinisme  de  la  sexualite  chez  V Hydatina  senta.  Compt.  rend. 
Acad.  Sci.,  CXIIL 

McClendon,  Dr.  J.  F. 

’09  Spermatogenesis  of  Pandarus  simatus.  Archiv  f.  Zell.  Forschung.,  V. 

’10  The  Effect  of  External  Conditions  upon  Reproduction  in  Daphnia.  Amer. 
Nat.,  July,  Vol.  44,  no.  523. 

Morgan,  Thos.  Hunt. 

’09  A  Biological  and  Cytological  Study  of  Sex  Determination  in  Phylloxerans 
and  Aphids.  Jour,  of  Exper.  Zool.,  Sept.,  Vol.  VTI.,  no.  2. 


94 


WYMAN  REED  GREEN. 


’14  Heredity  and  Sex.  Columbia  University  Press,  New  York. 

Nachtscheim,  Hans. 

’13  Cytologische  Studien  iiber  die  Geschlechtbestimmung  bei  der  Honigbiene, 
Apis  meliijica.  Aus  dem  Zoologischen  Institut  in  Miinchen.  Archiv  fiir 
Zellforschung. 

Nichols,  M.  Louise. 

’09  Comparative  Studies  in  Crustacean  Spermatogenesis.  Jour,  of  Morph., 
XX.,  pp.  461-478. 

Nussbaum,  M. 

’97  Die  Entstehung  des  Geschlechts  bei  Hydatina  senla.  Archiv  f.  mikr. 
Anat.,  XLIX. 

Ostwald,  W. 

’04  Experimentelle  Untersuchungen  iiber  den  Saisonpolymorphismus  bei 
Daphniden.  Archiv  f.  Entwickelungsmech.  d.  Organismen,  XVII. 

Papanicolau,  G. 

’10  Experiment.  Untersuch.  iiber  die  Fortpflanzungsverhaltnisse  der  Daphniden. 
Biol.  Centralblatt,  Bd.  XXX. 

’ll  Dazu  Anhang.  Ibid.,  Bd,  XXXI. 

Petrunkewitch,  Alex.  von. 

’00-01  Die  Richtungskorper  und  Schicksal  im  befruchteten  und  unbefruchteten 
Bienenei.  Zool.  Jahr.  Anat.  Ontog..  14. 

’02  Die  Schicksal  der  Richtungskorper  in  Drohnenei.  Ibid.,  17. 

Plainer,  G. 

’88  Die  erste  Entwicklung  befruchteter  und  parthenogenetischer  Eier  von 
Liparis  dispar.  Biol.  Centralblatt,  Bd.  VIII,,  p.  521, 

Popoff,  M. 

’07  Depression  der  Protozoenzelle  und  der  Geschlechtszellen  der  Metazoen. 
Archiv  fiir  Protistenk.  Supplementband.  (Festband  fiir  R.  Hertwig.) 

Punnett,  R.  C. 

’06  Sex  Determination  in  Hydatina  with  some  Remarks  on  Parthenogeno- 
genesis.  Proc,  Royal  Soc.,  B,  LXXVIII. 

Samassa,  P. 

’91  Untersuchungen  iiber  das  Centralnervensystem  der  Cladoceren.  Archiv  f. 
Mikr.  Anat.,  Bd.  38,  p.  100. 

’93  Keimblatterbildung  bei  den  Cladoceren,  I.  und  II.  Ibid.,  XLI,  Die 
Keimblatterbildung  bei  Moina.  Zool.  Anz.,  XVI. 

Scharfenberg,  U.  v. 

’ll  Studien  und  Experimente  iiber  die  Eibildung  und  den  Generationzyklus 
von  Daphnia  magna.  Internat.  Revue  der  Gesampten  Hydrobiologie 
und  Hydrographie.  Bd.  III.,  Biolog.  Suppl.,  Bd.  I. 

Schmankewitsch,  v.  W. 

’77  Zur  Kenntnis  des  Einflusses  der  ausseren  Lebensbedingungen  auf  die 
Organisation  der  Tiere.  Zeit.  Wiss.  Zool.,  Bd.  XXIX. 

Shull,  A.  F. 

’lo-ii  Studies  in  the  Life  Cycle  of  Hydatina  senta.  I,  and  II.  Jour,  of 
Exper.  Zool. 

Strauss,  Druckheim,  H. 

’19-20  Memoire  sur  les  Daphnia  de  la  classe  des  crustaces.  Memoires  du 
museum  d’hist.  Nat.  Paris,  Tome  V.,  VI.,  VII. 


LIFE  CYCLE  OF  SIMOCEPHALUS  VETULUS. 


95 


Strohl,  H. 

’o8  Polyphemusbiologie,  Cladocereneier  und  Kernplasmsrelation.  Internat. 
Rev.  ges.  Hydrobiol.  und  Hydrogr.,  Bd.  I.,  pp.  821-832. 

Vollmer,  C.  v. 

’12  Ueber  die  Entwickelung  der  Dauereier  der  Cladoceren.  Biol.  Centralblatt., 

■  Bd.  XXXII.,  Zahl  2. 

Warren,  Ernest. 

’00  On  the  Reaction  of  Daphnia  to  Certain  Changes  in  the  Environment. 
Quart.  Jour,  of  Micro.  Sci.,  p.  199. 

Weismann,  August. 

’77  Beitrage  zur  Xaturgeschichte  der  Daphnoiden,  II-IV.  Zeitschr.  f.  Wiss. 
Zool.,  XXVIIL,  pp.  93-254. 

’79  Beitrage  zur  Xaturgeschichte  der  Daphnoiden.  VI.,  Samen  und  Begattung 
der  Daphniden.  VI L,  Die  Entstehung  der  cyklischen  Fortpflanzung  bei 
den  Daphnoiden.  Zeitschr.  f.  wiss.  Zook,  33,  pp.  55-270. 

’80  Parthenogenese  bei  den  Ostracoden.  Zool.  Anz.,  Bd.  III.,  p.  82. 

’87  Die  Eibildung  bei  den  Daphnoiden.  Zeitschr.  f.  wiss.  Zool.,  Bd.  28,  p.  122. 
Weismann  und  Ischikawa. 

’89-91  Ueber  die  Paracopulation  in  Daphnoiden  Ei  sowei  iiber  Reifung  und 
Befruchtung  Deselben.  Zool.  Jahr.  Anat.,  Bd.,  pp.  155-196. 
Wesenberg-Lund,  H. 

’10  Grunziige  der  Biologie  und  Geographie  des  Suszwasserplanktons,  nebst 
Bemerkungen  iiber  Hauptprobleme  zukiinftiger  Limnologischer  Fos- 
schungun.  Biologisches  Supplement,  Bd.  I.,  Erste  Serie.  Zur  Inter- 
nationalen  Revue  der  Gesampten  Hydrobiologie  und  Hydrographie. 
Bd.  III. 

Whitney,  D.  D. 

’07  Determination  of  sex  in  Hydatina  senla.  Jour,  of  Exper.  Zool.,  V. 
Woltereck,  R. 

’08  Ueber  natiirliche  und  kiinstliche  Variatenbildung  bei  Daphniden.  Verh. 
der  Zool.  Gesellschaft,  pp.  234-240. 

’09  Weitere  Experimentelle  Untersuchungen  iiber  Artveranderung  speziell 
iiber  das  Wesen  quantitativer  Artunterscheide  bei  Daphniden.  Verhandl. 
d.  Deutsch.  Zoolog.  Gesellsch.,  pp.  1 10-172. 

’ll  Beitrag  Analyse  der  Vererbung  erworbener  Eigenschaften.  Transmutation 
und  Prainduction  bei  Daphnia.  Ibid.,  pp.  141-172. 

Woodruff,  L.  L. 

’14  On  the  So-called  Conjugating  and  Xon-conjugating  Races  of  Paramecium. 
Jour,  of  Exper.  Zool.,  XVI. 


1 


